Dyes, dye-sensitized solar cells, and methods of making and using the same

ABSTRACT

Provided herein are dyes, dye-sensitized solar cells, and sequential series multijunction dye-sensitized solar cell devices. The dyes include an electron deficient acceptor moiety, a medium electron density π-bridge moiety, and an electron rich donor moiety comprising a biaryl, a substituted biaryl, or an R 1 , R 2 , R 3  substituted phenyl where each of R 1 , R 2 , and R 3  independently comprises H, aryl, multiaryl, alkyl substituted aryl, alkoxy substituted aryl, alkyl substituted multiaryl, alkoxy substituted multiaryl, OR 4 , N(R 5 ) 2 , or a combination thereof, each R 4  independently comprises H, alkyl, aryl, alkyl substituted aryl, alkoxy substituted aryl, or a combination thereof; and each R 5  independently comprises aryl, multiaryl, alkyl substituted aryl, alkoxy substituted aryl, alkyl substituted multiaryl, alkoxy substituted multiaryl, or a combination thereof. The solar cells include a glass substrate, a dye-sensitized active layer, and a redox shuttle. The devices include at least two dye-sensitized solar cells connected in series.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/779,569, filed Jan. 31, 2020, which is a continuation-in-part of U.S.application Ser. No. 16/634,050, filed Jan. 24, 2020, which is anational stage application, filed under 35 U.S.C. § 371, ofInternational Patent Application No. PCT/US2018/043862, filed Jul. 26,2018, which claims the benefit of U.S. Provisional Application Ser. No.62/537,208, filed Jul. 26, 2017, and 62/547,981, filed Aug. 21, 2017,the entire disclosures of which are incorporated herein by thisreference.

GOVERNMENT INTEREST

This invention was made with government support under grant numbers1539035 and 1455167 awarded by the National Science Foundation (NSF),and DE-SC0019131 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

The present invention relates to dyes, dye-sensitized solar cells, andmethods of making and using the same. In particular, thepresently-disclosed subject matter relates to sensitizer dyes,sequential series multijunction dye-sensitized solar cells (SSM-DSC),and methods of making and using the same.

BACKGROUND

Global energy demand is expected to increase from 18TW in 2013 to 24-26TW in 2040, along with corresponding increase in CO₂ emissions, due toinevitable increase in population and industrialization. So far most ofthe energy (˜80%) has been derived from fossil fuels, which is notsustainable and detrimental to the environment. Therefore, sustainablefossil fuel-free pathways to electricity and fuel production, such asconversion of water to H₂, nitrogen to ammonia, and/or CO₂ to carbonsfuels with only 02 as a byproduct, are highly desirable.

While solar energy may be the most abundant energy source, efficientsolar energy capture, conversion, and storage (potentially in chemicalbonds) must be realized in order to provide a large scale, costeffective, carbon neutral energy source. Despite intensive researchefforts over the last few decades, such photovoltaic powered solar fuelsystems are currently lacking. For example, one highly studied type ofsolar cell includes perovskite solar cells (PSCs), which have beenreported with record efficiencies for powering water splitting and CO₂reduction catalysts. However, the stability of PSCs currently limitslong term generation of fuels from these systems, while their watersoluble lead-based nature raises various environmental and healthconcerns.

One possible alternative to PSC powered solar fuel systems includesdye-sensitized solar cells (DSCs). DSCs are promising in terms of cost,stability, commercial viability, and environmental impact when comparedwith many solar cell technologies. Additionally, DSCs are anexceptionally attractive solar cell technology for efficientlyconverting high potential energy photons. Existing research efforts forDSCs have attempted to increase efficiency of single-junction devicesprimarily through balancing both photocurrent (J_(sc)) and Voc.

With respect to achieving high V_(oc), Co(bpy-pz)₂ ^(3+/2+) was firstreported in 2011 by Gratzel et al. as a legitimate alternative to I⁻/I₃⁻ and Co(bpy)₃ ^(3+/2+) for DSCs with Y123 sensitizer. The highestV_(oc) provided by these DSCs was 1.0 V, with a PCE of up to 10%. TheV_(oc) was further improved to 1.1 V with custom made TiO₂ paste andatomic layer deposition. Additionally, Hagfeldt and Boschloo's groupreported D35 giving V_(oc) of 1.02 V and overall PCE of 3.6% withCo(bpy-pz)₂ ^(3+/2+), as well as V_(oc) of 0.85 V and overall PCE of6.5% with Co(bpy)₃ ^(3+/2+) under the reported conditions. However,single cell devices convert all absorbed photons regardless of potentialenergy to the same voltage, resulting in significant energetic wastefrom high-energy solar spectrum photons. As such, the highest V_(oc)values for a single DSC cell, as reported by Hanaya et al., arecurrently 1.45 V and 1.21 V with a tailored organic dye and Br₃ ⁻/Br⁻redox shuttle.

One strategy for increasing solar cell power conversion efficienciesincludes using series tandem DSCs (ST-DSCs) in conjunction withinorganics (e.g., GaAs or silicon) and/or organic photovoltaic (OPV)cells. This strategy relies on light absorbing materials of differentwavelength ranges, or photons of different potential energies, inseparate subcells to divide up the solar spectrum between these subcells(FIG. 1A). In contrast to parallel tandem DSCs, where the J_(sc) of theindividual devices is added, ST-DSCs provide addition of the V_(oc) fromindividual devices to provide a higher overall V_(oc). There are twomain approaches that can be adopted to achieve high V_(oc) in ST-DSCs,one includes mechanically stacked devices with n*2 terminals (n is thenumber of sub-cells) and the other includes mono-lithic two terminalcells. Using these approaches, ST-DSC V_(oc) values in the range of1.4-1.85 V have been reported with organic dyes in conjunction withinorganics and porphyrin sensitizers. DSCs in conjunction with othertype solar cells exhibited high voltages of 1.36 V with DSC/Silicon,1.85 V with DSC/GaAs, and 1.2 V with DSC/OPV.

This is particularly attractive for powering electrochemical cells,where V_(oc) values of 1.83 and 3.0 V were demonstrated to function forwater oxidation and CO₂ reduction, respectively. In fact, 1.83 Vorganic/porphyrin dye tandem DSCs have been successfully employed forwater oxidation along with 0.248 mV bias resulting in 7.4% applied biasphoton-to-current efficiency (ABPE) and solar to hydrogen (STH)efficiency of 5.75%. Although this represents an increase inphotovoltage over a single DSC cell, such device architectures have notdemonstrated a photovoltage in excess of 1.85 V from a singleilluminated area to date. In addition, existing ST-DSC devices have beenlimited to two subcell (double junction) systems, because efficientlight harvesting was expected to diminish beyond this point.

Accordingly, there is a need for devices and methods that provideincreased overall efficiency while generating high photovoltages from asingle illuminated area.

SUMMARY

This summary describes several embodiments of the presently-disclosedsubject matter, and in many cases lists variations and permutations ofthese embodiments. This summary is merely exemplary of the numerous andvaried embodiments. Mention of one or more representative features of agiven embodiment is likewise exemplary. Such an embodiment can typicallyexist with or without the feature(s) mentioned; likewise, those featurescan be applied to other embodiments of the presently-disclosed subjectmatter, whether listed in this summary or not. To avoid excessiverepetition, this summary does not list or suggest all possiblecombinations of features.

The presently-disclosed subject matter provides, in some embodiments, adye including an electron deficient acceptor moiety, a medium electrondensity π-bridge moiety, and an electron rich donor moiety comprising abiaryl, a substituted biaryl, or a moiety having the followingstructure:

where each of R¹, R², and R³ independently comprises H, alkyl, aryl,multiaryl, fused aryl, fused multiaryl, alkyl substituted aryl, alkoxysubstituted aryl, alkyl substituted multiaryl, alkoxy substitutedmultiaryl, OR⁴, or a combination thereof; and where each R⁴independently comprises H, alkyl, aryl, alkyl substituted aryl, alkoxysubstituted aryl, or a combination thereof.

In some embodiments, the donor moiety comprises:

andwhere n is between 1 and 30.

In some embodiments, the dye comprises:

In some embodiments, the substituted biaryl comprises an alkyl or alkoxysubstituted biaryl. In one embodiment, the alkyl substituted biarylcomprises the structure.

wherein n is between 1 and 30. In one embodiment, the alkoxy substitutedbiaryl comprises the structure:

where n is between 1 and 30.

In some embodiments, the π-bridge moiety comprises:

or derivatives thereof;where each X independently comprises H, F, CN, or CO₂R; and each Rindependently comprises H, alkyl, aryl, alkyl substituted aryl, alkoxysubstituted aryl, or a combination thereof.

In some embodiments, the acceptor moiety comprises:

or derivatives thereof,where each R independently comprises H, alkyl, aryl, alkyl substitutedaryl, alkoxy substituted aryl, or a combination thereof.

Also provides herein, in some embodiments, is a sequential seriesdye-sensitized solar cell including three dye-sensitized subcellsmechanically stacked and connected in series, each of the subcellsincluding a glass substrate coated with a transparent conductive film, adye-sensitized active layer, and a redox shuttle. In some embodiments,the dye-sensitized active layer of at least the first subcell issensitized with a dye comprising the dye of claim 1, D35, B11, Y123, orBlack Dye. In some embodiments, the dye-sensitized active layer issensitized with a dye comprising D35, Y123, RR9, or a dye including anelectron deficient acceptor moiety, a medium electron density π-bridgemoiety, and an electron rich donor moiety comprising a biaryl, asubstituted biaryl, or a moiety having the following structure:

where each of R¹, R², and R³ independently comprises H, alkyl, aryl,multiaryl, fused aryl, fused multiaryl, alkyl substituted aryl, alkoxysubstituted aryl, alkyl substituted multiaryl, alkoxy substitutedmultiaryl, OR⁴, or a combination thereof; and where each R⁴independently comprises H, alkyl, aryl, alkyl substituted aryl, alkoxysubstituted aryl, or a combination thereof. In some embodiments, theredox shuttle is a cobalt redox shuttle.

In some embodiments, the dye-sensitized solar cell further includes a UVcutoff filter. In one embodiment, the dye-sensitized active layer of thefirst subcell has a thickness of between 0.8 and 3.0 m. In anotherembodiment, the dye-sensitized active layer of the second subcell has athickness of between 1.5 and 5.0 m. In some embodiments, thedye-sensitized solar cell further includes an antireflective coatingapplied over a non-conductive side of at least one of a photoanode and acathode of one or more of the dye-sensitized solar cells. In someembodiments, the dye-sensitized solar cell further includes an immersionoil positioned between at least two of the subcells.

In some embodiments, the dye-sensitized active layer of the firstsubcell is sensitized with a dye comprising:

In some embodiments, the dye-sensitized active layer of at least thefirst and second subcells is sensitized with a dye comprising the dye ofclaim 1, D35, B11, Y123, or Black Dye. In some embodiments, thedye-sensitized active layer of each of the subcells is sensitized with adye comprising the dye of claim 1, D35, B11, Y123, or Black Dye.

Further provided herein, in some embodiments, is a sequential seriesmultijunction dye-sensitized solar cell including at least twodye-sensitized subcells mechanically stacked and connected in series,each of the subcells including a glass substrate coated with atransparent conductive, a dye-sensitized active layer, and a redoxshuttle, and at least one additional subcell mechanically stacked andconnected in series, where the at least one additional subcell is aninorganic solar cell. In some embodiments, the dye-sensitized activelayer of the first subcell has a thickness of between 0.8 and 3.0 m. Insome embodiments, the dye-sensitized active layer of the second subcellhas a thickness of between 1.5 and 5.0 m.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently-disclosed subject matter will be better understood, andfeatures, aspects and advantages other than those set forth above willbecome apparent when consideration is given to the following detaileddescription thereof. Such detailed description makes reference to thefollowing drawings, wherein:

FIGS. 1A-I show graphs and images illustrating SSM-DSC systems. (A)Shows a graph illustrating how solar spectrum photons were divided inprior work. (B) Shows the general structure for metal-free organicsensitizers. (C) Shows the structure of metal-free organic sensitizerD35. (D) Shows the structure of metal-free organic sensitizer Y123. (E)Shows the structure of the cobalt redox shuttle Co(bpy-pz)₂ ^(3+/2+).(F) Shows the structure of the cobalt redox shuttle Co(bpy)₃ ^(3+/2+).(G) Shows the structure of the surface capping agents pivalic acid,heptanoic acid, myristic acid, stearic acid, melisic acid,decylphosphonic acid, octadecylphosphonic acid, ethyltrimethoxy silane,isooctyltrimethoxy silane, octyltrimethoxy silane, dodecyltrimethoxysilane, octadecyltrimethoxy silane, perfluorooctyltrimethylsilane (PFTS;3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl-trimethoxysilane),3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyl-trimethoxysilane,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl-trimethoxysilane,3,3,4,4,5,5,6,6,6-nonafluorohexyl-trimethoxysilane,3,3,4,4,5,5,5-heptafluoropentyl-trimethoxysilane,3,3,4,4,4-pentafluorobutyl-trimethoxysilane, and3,3,3-trifluoropropyl-trimethoxysilane. (H) Shows the structure ofRu-sensitizer HD-2-mono. (I) Shows a graph illustrating energetic andtheoretical maximum possible values of D35, Y123, and HD-2-mono whenpaired with I⁻/I₃ ⁻, Co(bpy)₃ ^(3+/2+), and Co(bpy-pz)₂ ^(3+/2+). D35was measured on TiO₂. Y123 energetics are reported previously. HD-2-monowas measured in DMF solution. Redox shuttle values are measured in MeCN.

FIGS. 2A-B show schematics illustrating the synthesis of a cobalt redoxshuttle. (A) Shows a schematic illustration of a method of synthesizinga bpy-pz ligand according to an embodiment of the disclosure. (B) Showsa schematic illustration of a method of synthesizing Co(bpy-pz)₂^(3+/2+) using the ligand of (A).

FIG. 3 shows images of an Au₂O₃ electrode before (left) and after(right) anodization.

FIG. 4 shows a graph illustrating the UV-Vis absorption profile of D35and Y123 in DCM with HD-2-mono in DMF.

FIG. 5 shows graphs illustrating IV curves (left) and IPCE (right) ofoptimized Co(bpy-pz)₂ ^(3+/2+) based devices in Table 1 for D35 (solidlines) and Y123 (dotted lines).

FIG. 6 shows graphs illustrating photovoltage transient measurementsplotted as electron lifetime versus open-circuit voltage and chargedensity versus open-circuit voltage for Co(bpy-pz)₂ ^(3+/2+) (top) andCo(bpy)₃ ^(3+/2+) (bottom).

FIGS. 7A-C show graphs illustrating IV results for various SSM-DSCdevices. (A) Shows IV curve results for single illuminated area, 2-5SSM-DSC devices (top). IV curves for 2 SSM-DSC systems (4+4 or 4+5subcells) irradiated side-by-side (bottom). (B) Shows graphsillustrating IV results for tandem devices left (two and three devices),right (four and five devices) given in Table 3 for Co(bpy-pz)^(3+/2+)electrolyte based devices. (C) Shows IV graphs for four and five deviceswith two sets (set1 and set2) which were connected in series to add upphotovoltage.

FIG. 8 shows IPCE graphs for individual subcells in the SSM-DSCconfiguration for Co(bpy-pz)₂ ^(3+/2+) electrolyte in Tables 5-7. Bluecolored dotted line in each graph represent the curve as the result ofmanual addition of all the IPCE's from devices in the stack.

FIGS. 9A-B show graphs illustrating (A) IV and (B) IPCE curvescorresponding to devices reported in Table 6 comparing varying only thethird device.

FIG. 10 shows graphs illustrating IV (left) and IPCE (right) of Co(bpy)₃^(3+/2+) electrolyte based devices in Table 8.

FIG. 11 shows a graph illustrating IV results for SSM-DSC devices givenin Table 9 for Co(bpy)₃ ^(3+/2+) electrolyte based devices.

FIG. 12 shows a graph illustrating IV results for a 5 subcell SSM-DSCdevice (the final entry in Table 10) with the Co(bpy)₃ ^(3+/2+) redoxshuttle measured under different % sun values.

FIG. 13 shows graphs illustrating IV results for a three subcell SSM-DSCdevice and each subcell in the SSM-DSC configuration before electrolysis(solid) and after electrolysis (dotted). Left: A 1.2 μm D35 front devicewas used for enhanced current matching through each subcell with aslightly lower overall voltage. Right: A 2.2 μm D35 front device wasused for a higher overall voltage with a slight lower current due tomismatching of the subcell current flow. The IV curve for the Au₂O₃/IrO₂EC system is shown as well with the marked intersection of the IV curve.The left hand case has the EC curve intersection of the SSM-DSC curvecloser to the maximum power point of the SSM-DSC (marked as a blue dotwith no intersecting line) leading to a higher overall solar-to-fuelefficiency. The right hand case shows a device with enhanced stabilityof the front cell.

FIG. 14 shows graphs illustrating PV-EC IV results for a 3 subcellSSM-DSC, blue dot highlights the operating point for a PV-EC device(top). J_(op) and FE as a function of time for the PV-EC system(bottom).

FIG. 15 shows a graph illustrating current flow comparison through aPV-EC cell with different front cells.

FIG. 16 shows graphs illustrating demonstration of a 1.2 μm stable frontdevice by the application of a 385 nm UV cutoff filter and gelelectrolyte. Black (triangle): gel electrolyte with the filter; Red(cross): liquid electrolyte with filter; Blue (square): liquidelectrolyte no filter; Green (diamond): gel electrolyte no filter.

FIG. 17 shows an image illustrating PV-EC setup.

FIG. 18 shows an image illustrating SSM-DSC which was dropped in H₂Oitself.

FIG. 19 shows a schematic of a method of synthesizing(E)-2-cyano-3-(4-(7-(2,2′,2″,4,4″-pentakis(hexyloxy)-[1,1′:3′,1″-terphenyl]-5′-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylicacid coded as RR9.

FIG. 20 shows a schematic of a method of synthesizing4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)benzaldehyde.

FIG. 21 shows a graph illustrating the absorption curve of RR9 indichloromethane.

FIGS. 22A-B show chemical structures of (A) RR9 and (B) Fe(bpy)₃^(+2/+3) redox mediators.

FIG. 23 shows a graph illustrating the IPCE spectrum of RR9 withFe(bpy)₃ ^(3+/2+).

FIG. 24 shows a graph illustrating IV curves of electrodes 5.3 micronsTiO₂+5 microns of scattering layer dipped for 30 min (0.5 M solution)while N₂ was flowing (blue curve) and the best device with the samethickness electrodes with no N₂ flow (red curve).

FIG. 25 shows a graph illustrating IV curves of electrode 2.7 micronsTiO₂+5 microns of scattering layer dipped for 4 hrs in a RR9 solution(0.5 M solution).

FIGS. 26A-B show graphs illustrating IPCE curves for RR9. (A) Shows agraph illustrating IPCE curve of electrodes 2.7 microns TiO₂+5 micronsof scattering layer dipped for 4 hrs in a RR9 solution (0.5 M solution).(B) Shows a graph illustrating IPCE curves for RR9 with Fe(bpy)₃^(3+/2+) and Co(bpy)₃ ^(3+/2+) shuttles.

FIG. 27 shows a graph illustrating Voc.

FIG. 28 shows J-V curves for RR9/Fe(bpy)₃ ^(3+/2+) based DSC and SSM-DSCdevices.

FIG. 29 shows structures for RR8, RR9, RR10, AP11, AP14, AP16, AP17,TD1, and TD2.

FIG. 30 shows structures for Y123, D35, XY1, and TD3.

FIG. 31 shows the structure for SM315.

FIGS. 32A-B show graphs illustrating absorption and oxidationspotentials for AP11, AP14, AP16, and AP17. (A) molar absorptivity versuswavelength. (B) oxidation potential.

FIG. 33 shows a graph illustrating IV results for AP11, AP14, AP16, andAP17.

FIG. 34 shows a schematic view of a method for donor synthesis.

FIG. 35 shows a schematic view of a method for synthesizing AP11, AP14,AP16, and AP17.

FIG. 36 shows a schematic comparison of SSM-DSC systems with and withoutCYTOP and immersion oil.

FIG. 37 shows the structure of CYTOP.

FIG. 38 shows the structure of B11.

FIG. 39 shows a graph illustrating IV for a D35/B11/B11 system withoutCYTOP or oil, a D35/B11/B11 system with CYTOP, and a D35/B11/B11 systemwith CYTOP and oil.

FIG. 40 shows an image and graph illustrating (left) a summary of designstudied in Example 13 and (right) comparison of normalized typicalincident photo-to-current conversion efficiency (IPCE) response of DSC,GaAs, and Si cells.

FIG. 41 shows graphs illustrating (Top) J-V and (Bottom) IPCE comparisonof individual DSC (B11, Black Dye, AP25/D35), GaAs, and Si devicesemployed in Example 13.

FIG. 42 shows a graph illustrating J-V comparison of three subcellsSSM-DSCs with third cell as either a DSC (B11, Black Dye, AP25/D35),GaAs, or Si.

FIG. 43 shows a graph illustrating comparison of third device IPCEresponse when in the stack of devices for SSM-DSCs.

FIG. 44 shows a graph illustrating PCE of devices as function of sunvalue for DSC (Black Dye, AP25/D35), GaAs, and Si.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described below in detail. Itshould be understood, however, that the description of specificembodiments is not intended to limit the disclosure. All modifications,equivalents, and alternatives falling within the spirit and scope of thedisclosure as defined by the appended claims are to be covered.

Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the disclosure belongs. Any methods and materialssimilar to or equivalent to those described herein can be used in thepractice or testing of the present disclosure, including the methods andmaterials are described below.

Following long-standing patent law convention, the terms “a,” “an,” and“the” refer to “one or more” when used in this application, includingthe claims. Thus, for example, reference to “a cell” includes aplurality of cells, and so forth.

The terms “comprising,” “including,” and “having” are intended to beinclusive and mean that there may be additional elements other than thelisted elements.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as reaction conditions, and so forth usedin the specification and claims are to be understood as being modifiedin all instances by the term “about.” Accordingly, unless indicated tothe contrary, the numerical parameters set forth in this specificationand claims are approximations that can vary depending upon the desiredproperties sought to be obtained by the presently-disclosed subjectmatter.

As used herein, the term “medium electron density” means any group ormoiety having less electron density than the donor region but more thanthe acceptor region. As will be understood by those skilled in the art,the range of suitable electron densities will depend upon the specificdonor and acceptor regions of the compounds disclosed herein.

As used herein, the term “about,” when referring to a value or to anamount of mass, weight, time, volume, concentration, percentage, or thelike is meant to encompass variations of in some embodiments ±50%, insome embodiments ±40%, in some embodiments ±30%, in some embodiments±20%, in some embodiments ±10%, in some embodiments ±5%, in someembodiments ±1%, in some embodiments ±0.5%, and in some embodiments±0.1% from the specified amount, as such variations are appropriate toperform the disclosed method.

As used herein, ranges can be expressed as from “about” one particularvalue, and/or to “about” another particular value. It is also understoodthat there are a number of values disclosed herein, and that each valueis also herein disclosed as “about” that particular value in addition tothe value itself. For example, if the value “10” is disclosed, then“about 10” is also disclosed. It is also understood that each unitbetween two particular units are also disclosed. For example, if 10 and15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

All combinations of method or process steps as used herein can beperformed in any order, unless otherwise specified or clearly implied tothe contrary by the context in which the referenced combination is made.

DETAILED DESCRIPTION

The details of one or more embodiments of the presently-disclosedsubject matter are set forth in this document. Modifications toembodiments described in this document, and other embodiments, will beevident to those of ordinary skill in the art after a study of theinformation provided in this document. The information provided in thisdocument, and particularly the specific details of the describedexemplary embodiments, is provided primarily for clearness ofunderstanding and no unnecessary limitations are to be understoodtherefrom. In case of conflict, the specification of this document,including definitions, will control.

The presently-disclosed subject matter relates to dyes, solar cells, andmethods for making and using the same. Provided herein, in someembodiments, is a dye for use in solar cells. For example, in oneembodiment, the dye includes a sensitizer dye for use in dye-sensitizedsolar cells (DSCs). In another embodiment, the dye includes adonor-π-bridge-acceptor (D-π-A) structure, where the donor moiety is anelectron rich region, the π-bridge moiety includes medium electrondensity, and the acceptor moiety is an electron deficient region.

In some embodiments, the donor moiety includes, but is not limited to,any moiety according to Structure I below:

where R¹, R², and R³ individually include, but are not limited to, H,alkyl, aryl, multiaryl, fused aryl, fused multiaryl, alkyl substitutedaryl, alkoxy substituted aryl, alkyl substituted multiaryl, alkoxysubstituted multiaryl, OR⁴, N(R⁵)₂, or a combination thereof; each R⁴individually includes, but is not limited to, H, alkyl, aryl, alkylsubstituted aryl, alkoxy substituted aryl, or a combination thereof; andeach R⁵ individually includes, but is not limited to, aryl, multiaryl,alkyl substituted aryl, alkoxy substituted aryl, alkyl substitutedmultiaryl, alkoxy substituted multiaryl, or a combination thereof.

In some embodiments, the donor moiety includes, but is not limited to,any moiety according to Structure II below:

where R¹, R², and R³ individually include, but are not limited to, H,alkyl, aryl, multiaryl, fused aryl, fused multiaryl, alkyl substitutedaryl, alkoxy substituted aryl, alkyl substituted multiaryl, alkoxysubstituted multiaryl, OR⁴, or a combination thereof; each R⁴individually includes, but is not limited to, H, alkyl, aryl, alkylsubstituted aryl, alkoxy substituted aryl, or a combination thereof.

Suitable donor moieties according to generic Structures I and II aboveinclude, but are not limited to:

Additionally or alternatively, in some embodiments, the donor moietyincludes a polycyclic moiety, such as, but not limited to, biarylmoiety, an unsubstituted biaryl, an alkyl substituted biaryl, or analkoxy substituted biaryl, any other suitable polycyclic moiety with twoor more fused rings, or any other suitable polycyclic moiety. In oneembodiment, the biaryl donor moiety includes:

In another embodiment, the donor moiety includes one or more of thefollowing:

In each of the donor moieties above, n is at least 1, between 1 and 30,between 1 and 20, between 1 and 15, between 1 and 10, between 1 and 6,or any combination, sub-combination, range, or sub-range thereof. Forexample, in one embodiment, n is 6, such that C_(n)H_(2n+1) andC_(n)H_(2n+1)O are C₆H₁₃ and C₆H₁₃O, respectively. In another example,the donor moiety is selected from the following group:

Each of the donor moieties disclosed herein above may individually becombined with any suitable π-bridge and acceptor moiety to form thesensitizer dye. Suitable π-bridge moieties include, but are not limitedto, any moiety according to one or more of the following structures:

or derivatives thereof. Each R group in the π-bridge moieties aboveindependently includes, but is not limited to, H, alkyl, aryl, alkylsubstituted aryl, alkoxy substituted aryl, or a combination thereof.Each X group in the π-bridge moieties above independently includes, butis not limited to, H, F, CN, or CO₂R. For example, such π-bridgemoieties include, but are not limited to:

or derivative there. Although the examples above focus on symmetricπ-bridge moieties, as will be understood by those skilled in the art,the disclosure is not so limited and includes dissymmetric π-bridgemoieties as well. Dissymmetric π-bridge moieties may include simpleanalogues of the above-mentioned symmetric moieties, in terms ofsynthesis and/or device performance. For example, as each X is chosenindependently of the other, one dissymmetric π-bridge moiety may includeF at one X and H, CN, or CO₂R at the other X. Similarly, in anotherexample, both X may include CO₂R with different R groups at each X.

In certain embodiments, the π-bridge moieties are selected based upon adesired electron density. While those skilled in the art will readily beable to determine the relative electron density and/or deficiency of theπ-bridge moieties through common knowledge and/or routineexperimentation, as an example, the following π-bridge moieties arelisted in order of increasing electron deficiency:

Suitable acceptor moieties include those known to individuals skilled inthe art, such as, but not limited to, any moiety according to thefollowing structure.

or a derivative thereof. Each R group in the acceptor moieties aboveindependently includes, but is not limited to, H, alkyl, aryl, alkylsubstituted aryl, alkoxy substituted aryl, or a combination thereof. Forexample, in some embodiments, the π-bridge-acceptor moiety with whichany of the donor groups disclosed herein may be combined includes, butis not limited to:

where each R group is as defined above.

In some embodiments, the sensitizer dyes according to generic StructureI or II, and including the donor, π-bridge, and acceptor moieties above,include one or more of the following:

When including the donor groups disclosed herein, the resultingsensitizer dyes provide improved photovoltaic properties as compared toexisting dyes and/or sensitizers. For example, in some embodiments, thedyes disclosed herein provide increased voltage in DSCs without TiO₂modification. More specifically, in one embodiment, the dyes provide avoltage of at least 1.0 V, at least 1.1 V. at least 1.2 V, at least 1.3V, at least 1.4 V, or any combination, sub-combination, range, orsub-range thereof, from a single junction system.

In some embodiments, alkylated donor moieties may be used for coursetuning of the dye ground-state oxidation potential. Additionally oralternatively, the use of multiple alkyl chains may decreasecounter-productive electron transfer pathways in devices.

Also provided herein, in some embodiments, is a solar cell deviceincluding one or more dye-sensitized solar cells (DSCs). In oneembodiment, the one or more DSCs are stacked within the solar celldevice, forming a sequential series multijunction (SSM)-DSC device. Insuch embodiments, each DSC may alternatively be referred to herein as asubcell. In another embodiment, the SSM-DSC device includes a singleilluminated area, where light travels sequentially through each of theDSCs in a stack. In a further embodiment, the voltage output of each DSCin the SSM device is additive, such that the overall photovoltage(V_(oc)) of the device is equal to the sum of the individual DSCvoltages forming the device.

The SSM-DSC devices disclosed herein include any suitable combination ofDSCs. Each DSC independently includes any suitable dye for use in asolar system. In one embodiment, at least one of the DSCs in the systemincludes one or more of the dyes disclosed herein. In anotherembodiment, all of the DSCs in the system include one or more of thedyes disclosed herein. Alternatively, at least one of the DSCs in thesystem may include one or more existing dyes.

In some embodiments, a dye is individually selected for each of theDSCs, such that the dye of any one DSC may be the same or different fromthe dye of any other DSC in the device. In one embodiment, the dye foreach DSC is selected based upon positioning of the DSC within the stack.In another embodiment, the DSCs higher in the stack, which light passesthrough first, may include different dyes from DSCs lower in the stack,which light passes through later in the series (i.e., after passingthrough at least one other DSC). For example, in one embodiment, DSCshigher in the stack include an orange dye, such as(E)-3-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylicacid (D35), while DSCs lower in the stack include a red dye, such as3-{6-{4-[bis(2′,4′-dihexyloxybiphenyl-4-yl)amino-]phenyl}-4,4-dihexyl-cyclopenta-[2,1-b:3,4-b′]dithiphene-2-yl}-2-cyanoacrylicacid (Y123). In another example, the SSM-DSC device includes threesubcells, with the first subcell including D35, the second subcellincluding B11, and the third subcell including Black Dye.

Additionally or alternatively, in some embodiments, the first subcellmay include a wide band-gap dye. Wide band-gap dyes include, but are notlimited to:

In such embodiments, the second subcell may include any other suitabledye disclosed herein, such as, but not limited to, D35 or Y123.Furthermore, a third subcell may include a dye such as, but limited to,Black Dye, or an inorganic solar cell. For example, a three subcellSSM-DSC device may include a wide band-gap dye in the first subcell, D35in the second subcell, and Black Dye in the third subcell. In someembodiments, this combination of dyes including a wide band-gap dye inthe first subcell increases the voltage output as compared to devicesusing D35 in the first subcell. Alternatively, the use of an inorganicsolar cell as the third subcell forms a hybrid SSM-DSC/inorganic solarcell device. Without wishing to be bound by theory, it is believed thatthe addition of the SSM-DSC subcells to form the hybridSSM-DSC/inorganic solar cell boosts the efficiency of existing solarcells. In addition, the device using the wide band-gap dye in the firstsubcell enable the use of alternate positively charged redox shuttles toFe.

As will be appreciated by those skilled in the art, the disclosure isnot limited to the specific dyes or arrangement described above, and mayinclude any other suitable combination and/or arrangement of dyes, suchas, but not limited to, dyes having peak absorption spectrums in therange of from 400 to 1200 nm, metal free organic dyes, RR9, or acombination thereof. Furthermore, each of the DSCs in the device mayalso include any suitable redox shuttle, with the redox shuttle of anyone DSC being the same or different from the redox shuttle of any otherDSC in the device. Suitable redox shuttles include, but are not limitedto, cobalt redox shuttles, such as [Co(bpy-pz)₂ ^(]3+/2+),[Co(bpy)₃]^(3+/2+); [Co(bpyNO₂)₃]^(3+/2+), and/or [Co(phen)₃]^(3+/2+);copper redox shuttles such as [Cu(dmby)₂]^(2+/1+) and/or[Cu(dmp)₂]^(2+/1+); ruthenium redox shuttles such as [Ru(bpy)₃]^(3+/2+);nickel redox shuttles such as [Ni(bpy(OMe)₂)₃]^(3+/2+); manganese redoxshuttles such as [Mn(bpy)₃]^(3+/2+); iron redox shuttles, such asFe(bpy)₃ ^(3+/2+); common derivatives thereof, or any suitablecombination thereof.

Additionally or alternatively, in some embodiments, a composition and/orthickness of any one DSC may be the same or different from thecomposition and/or thickness of any other DSC in the device. Forexample, in one embodiment, an active layer of the DSCs higher in thestack, such as a TiO₂ active layer, includes a decreased thickness ascompared to the active layer of DSCs lower in the stack. In anotherembodiment, DSCs lower in the stack include a scattering layer, such asa TiO₂ scattering layer, and/or DSCs higher in the stack are devoid of ascattering layer. In a further embodiment, a thickness of the scatteringlayer is up to about 4.5 μm.

Suitable thicknesses for the active layer of each DSC in the deviceinclude, but are not limited to, between about 0.5 and about 20 μm,between about 0.5 and about 15 μm, between about 0.5 and about 10 μm,between about 0.5 and about 5 μm, between about 0.5 and about 4.5 μm,between about 0.5 and about 4.0 μm, between about 0.5 and about 3.5 μm,between about 0.5 and about 3.0 μm, between about 0.5 and about 2.5 μm,between about 0.8 and about 3.0 μm, between about 0.8 and about 2.2 μm,between about 1.5 and about 5.0 μm, between about 1.5 and about 4.5 μm,between about 10 and about 20 μm, between about 10 and about 15 μm orany combination, sub-combination, range, or sub-range thereof. In someembodiments, the active layer of the first subcell includes a thicknessof between about 0.5 and about 3.0 μm, between about 0.8 and about 3.0μm, between about 0.8 and about 2.2 μm, or any combination,sub-combination, range, or sub-range thereof. In some embodiments, theactive layer of the second subcell includes a thickness of between about1.5 and about 5.0 μm, between about 1.5 and about 4.5 μm, or anycombination, sub-combination, range, or sub-range thereof. In someembodiments, the active layer of the third subcell includes a thicknessof between about 10 and about 20 μm, between about 10 and about 15 μm orany combination, sub-combination, range, or sub-range thereof. Forexample, in one embodiment, the active layer of the first subcellincludes a thickness of between about 0.8 and about 3.0 μm, the activelayer of the second subcell includes a thickness of between about 1.5and about 5.0 μm, and the active layer of the third subcell includes athickness of between about 10 and about 15 μm.

In some embodiments, the thickness of the active layer is controlledthrough paste formulations and screen mesh selection. In someembodiments, as compared to existing devices which utilize thickeractive layers, the active layers disclosed herein provide improvedperformance of the device. Additionally or alternatively, in someembodiments, the active layers disclosed herein are transparent orpartially transparent.

In some embodiments, the SSM-DSC devices disclosed herein include a UVcutoff filter. In certain embodiments, one or more of the DSCs include asurface capping layer. The surface capping layer, when present, forms aninsulating layer that slows the recombination of electrons in the activelayer with the redox shuttles. By slowing the recombination of electronswith the redox shuttles, the surface capping layer increases the V_(oc)of the device. In one embodiment, the surface capping layer includes afluorinated self-assembled mono layer (FSAM), such as, but not limitedto, perfluorooctyltrimethylsilane (PFTS). In another embodiment,inclusion of the surface capping layer is dependent upon the dyeselected for that particular DSC. For example, in a further embodiment,DSCs with D35 include a surface capping layer, while DSCs with Y123 aredevoid of a surface capping layer. Without wishing to be bound bytheory, it is believed that PFTS treatment of DSCs with Y123 resulted indye desorption from the TiO₂ surface, which decreased overallperformance of the DSC.

The DSCs disclosed herein may also include antireflective coatingsand/or immersion oils on the surfaces outside the DSC devices. Theantireflective coating, when included, is positioned and/or applied overthe non-conductive side of the photoanode and/or cathode/counterelectrode of one or more DSCs in the device. As used herein, the termphotoanode refers to the anode portion of each DSC, which is opposite acathode/counter electrode with respect to a substrate of the DSC, andwhen in use is positioned on a surface of the substrate that facestoward a light/energy source. In one embodiment, the antireflectivecoating includes a CYTOP layer formed over the photoanode of at leastone DSC in the device. In another embodiment, the antireflective coatingincludes a block copolymer polystyrene-block-poly(methyl methacrylate)(PS-b-PMMA). In another embodiment, the antireflective coating includesa polymer blend of polystyrene and poly(methyl methacrylate). Theimmersion oil with a similar refractive index to the substrate of theDSC devices, when included, is positioned between the counter electrodeof one DSC and the photoanode of another DSC.

In some embodiments, the use of an antireflective coating and/or animmersion oil changes the interfacial refractive index to minimize lightlosses. Additionally or alternatively, in some embodiments, the use ofan antireflective coating and/or immersion oil provides an increase inoverall power conversion efficiency (PCE). For example, in oneembodiment, the use of an antireflective coating and an immersion oil ina 3 SSM-DSC system provides an overall PCE of greater than 10% withoutput of greater than 2 V, which represents a substantial increase ascompared to other 3 SSM-DSC systems not including antireflectivecoatings and/or immersion oils.

By employing the dyes, compositions, thicknesses, redox shuttles,surface capping layers, antireflective coatings, and/or immersion oilsdisclosed herein, the instant DSCs provide increased photovoltage and/orpower conversion efficiency (PCE) as compared to existing DSCs, whilemaintaining and/or increasing photocurrent and fill factor. For example,in contrast to existing series tandem DSCs, which are limited to aphotovoltage of less than 1.9 V from two solar cells, the instant DSCsprovided a double junction photovoltage from a single area illuminateddevice of 1.93 V. Additionally, in contrast to existing series tandemdevices, which have been limited to two DSCs due to a lack of lightharvesting with additional cells, the DSCs disclosed herein may beselected to divide the usable solar spectrum between cells and permitthe inclusion of three or more DSCs in a single stack. For example, theSSM-DSC devices disclosed herein may include 2, 3, 4, 5, 6, 7, 8, 9, 10,or more devices in a single stack. Furthermore, light harvesting of thelower cells in devices with three or more DSCs may be increased throughsequential active layer thickness modulation and dye selection.

This increased number of DSCs in a single stack and/or increased lightharvesting from lower cells in such stacks further increases the overallphotovoltage of the device, providing photovoltages not previouslypossible with existing devices. For example, in one embodiment, a triplejunction (i.e., three solar cell) device provides a photovoltage of atleast about 2.8 V with an overall PCE of at least about 7%, which issufficient to potentially power water oxidation and CO₂ reductionwithout the need of external bias. In another embodiment, a fourjunction (i.e., four solar cell) device with a D35 sensitized first cellhaving a TiO₂ active layer thickness of about 0.8 μm, followed by a D35sensitized second cell with a TiO₂ active layer thickness of about 2.2μm, a Y123 sensitized third cell with a TiO₂ active layer thickness ofabout 2.2 μm, and a Y123 sensitized fourth cell with a TiO₂ active layerthickness of about 4.5 μm and a TiO₂ scattering layer thickness of about4.5 μm provides a photovoltage of at least about 3.75 V with an overallPCE of at least about 6%. In a further embodiment, a five junction(i.e., five solar cell) device provides a photovoltage of at least about4.7 V with a PCE of about 4.3%, while maintaining a photocurrent densityof about 2.4 mA/cm².

In some embodiments, two or more SSM-DSC devices may be combined in aside-by-side tandem configuration to further increase the overallphotovoltage. In one embodiment, for example, a side-by-side SSM-DSCconfiguration (i.e., a double illuminated area device) provides aphotovoltage of at least about 8.5 V with a PCE of at least about 4%. Aswill be appreciated by those skilled in the art, the SSM-DSC devicesdisclosed herein are not limited to the photovoltages and PCE valuesprovided above, as varying the SSM-DSC device configurations willprovide different photovoltages. For example, the photovoltagesdiscussed above may be increased by application of efficient red photonharvesting sensitizers (>750 nm), connecting SSM-DSC devices in tandemwith inorganic PV's (Silicon, GaAs, etc.), reducing glass relatedtransmission losses (e.g., application of low reflectance glasscoatings), incorporation of colorless redox shuttles with theoreticalVo, values greater than 1.5 V for a single cell, optimization ofmaterials for differing spectral regions, or combinations thereof.Accordingly, in certain embodiments, the single stacked system providesa photovoltage of up to about 10 V or more, and the two SSM-DSC devicesin a side-by-side tandem configuration provides a photovoltage of up toabout 20 V or more. Practical conversion efficiencies of at least about15% are also contemplated by the SSM-DSC devices disclosed herein.

Also provided herein are methods of using the SSM-DSC devices disclosedabove. One such method includes powering battery charging with theSSM-DSC devices disclosed herein, such as those batteries typicallyrequiring an external source with 3.7 V to 7.0 V. Another methodincludes employing the SSM-DSC devices disclosed herein to powersolar-to-fuel conversion. For example, in one embodiment, the unassisted(non-potential biased) SSM-DSC devices disclosed herein provide the atleast 1.83 V photovoltage required to power the electrolysis of H₂O(i.e., conversion of H₂O to H₂ fuel). In contrast to existing devicesthat do not observe O₂ production, both half reactions occur during theelectrolysis of water with the SSM-DSC devices disclosed herein. Inanother embodiment, the unassisted (non-potential biased) SSM-DSCdevices disclosed herein provide the at least 3.0 V photovoltagerequired to power the electrolysis of CO₂ (i.e., conversion of CO₂ tothe gasoline precursor CO). In a further embodiment, in contrast toPerovskite systems, which require the use of a constant flow argonchamber, the instant devices provide H₂O and CO₂ reduction when open toair and/or fully submerged in water. Additionally or alternatively, dueto increased voltage per area as compared to existing solar celltechnology, the SSM-DSC devices disclosed herein may be employed toprovide power for a host of applications where surface areas arelimited, such as on clothing, automobiles, portable solar cells, etc.

Further provided herein are methods of forming the instant DSCs andSSM-DSC devices. In some embodiments, forming the DSCs includes sealinga photoanode and a counter electrode, then adding an electrolyte to thecounter electrode and securing contacts to the DSC. In one embodiment,the photoanode and counter electrode are sealed with a hot melt gasketby heating the system under pressure. For example, in anotherembodiment, the photoanode and counter electrode are sealed with a 10-60μm thick hot melt gasket by heating the system at 130° C. under apressure of 0.2 psi for 1 minute. In a further embodiment, theelectrolyte is added through a hole in the counter electrode, which issubsequently sealed once the electrolyte has been added. The contactsare secured to the DSC by any suitable method, such as, but not limitedto, soldering.

In some embodiments, the photoanode and/or counter electrode areprepared prior to forming the DSC. In one embodiment, preparing thephotoanode includes providing a substrate, pre-treating the substrate,applying an active layer to the substrate, optionally applying ascattering layer to the substrate, and sensitizing the substrate. Thesubstrate includes any suitable material for use in a photoanode,including, but not limited to, glass, plastics, textiles, and/or anyother suitable material. In some embodiments, the glass or othersubstrate is coated with a transparent conducting film (TCF), such asfluorine doped tin oxide (FTO). A suitable sheet resistance for thesubstrate includes between about 5 and about 25 Ω/sq. sheet, betweenabout 5 and about 20 Ω/sq. sheet, between about 10 and about 15 Ω/sq.sheet, or any suitable combination, sub-combination, range, or sub-rangethereof. For example, in one embodiment, FTO coated glass substratesthat will ultimately include a scattering layer have a sheet resistanceof 10 Ω/sq. sheet, and FTO coated glass substrates that will not includea scattering layer have a sheet resistance of 15 Ω/sq. sheet.

In certain embodiments, pre-treating the substrate includes cleaning thesubstrate and/or applying an underlayer to the substrate. In oneembodiment, cleaning the substrate includes one or more of submergingand sonicating the substrate in an aqueous solution, rinsing thesubstrate with water, sonicating the substrate in acetone, sonicatingthe substrate in ethanol, and/or UV/ozone cleaning the substrate. Inanother embodiment, applying an underlayer to the substrate, such as aTiO₂ underlayer, includes submerging the substrate (conductive side up)in a TiCl₄ solution in water, followed by heating the submergedsubstrate. After heating, the substrate is rinsed first with water thenwith ethanol.

Next, in some embodiments, applying the active layer to the substrateincludes screen printing a mesoporous layer of a desired thickness onthe substrate. In one embodiment, the mesoporous layer includes a TiO₂layer with a particle size of about 30 nm. In another embodiment, thethickness of the mesoporous layer is at least partially determined bythe properties of the screen selected for the screen printing.Additionally or alternatively, the thickness may be adjusted bymodifying the consistency of the material prior to applying. Modifyingthe consistency of the material may include diluting the material priorto applying. For example, the thickness of the applied layer may bereduced by diluting a P30 TiO₂ paste with terpineol. Applying theoptional scattering layer includes screen printing of a material with anincreased particle size as compared to the active layer. Suitableparticle sizes for the scattering layer material include, but are notlimited to, at least about 100 nm.

After applying the active layer, the substrate is sintered withprogressive heating. In one embodiment, sintering the substrate includesprogressively heating the substrate from about 125° C. to about 500° C.with various ramp speeds and/or hold lengths between temperatures.Following sintering, the photoanode is soaked in a TiCl₄ water solutionand heated to about 500° C. Subsequent sensitization of the photoanodeincludes immersing the active layer film in a dye solution, such as, butnot limited to, a D35 or Y123 dye solution.

Preparing the counter electrodes includes forming one or more holesthrough the insulating side to the conductive side of a substratematerial. For example, in one embodiment, preparing the counterelectrodes includes drilling two holes through the insulating side tothe conductive side of an FTO coated glass substrate. In anotherembodiment, the FTO coated glass substrate of the counter electrodeincludes a resistance of 7 Ω/sq. sheet for full subcells and 15 Ω/sq.sheet for all other subcells. In a further embodiment, for platinumelectrodes, a thin layer of Pt-paste is slot printed with a punchedpiece of tape on the conductive side, followed by heading of theelectrode at a set temperature for a predetermined amount of time.

These solution processable solar cells provide comparable manufacturingpotentials as compared to PSC and OPV solar cells without the decreasedvoltage of PSC cells or the internal losses and decreased efficiency ofOPV cells. Additionally, the DSCs disclosed herein can be formed withoutstrictly controlled atmosphere (e.g., open to air), including conditionsthat would render OPVs and PSCs inoperable.

As further discussed in the Examples below, the DSCs disclosed hereinprovide ease of manufacture through solution processes, high performanceunder low light conditions, and/or variety in terms of devices color andmaterials on which the solar cells can be built.

The presently-disclosed subject matter is further illustrated by thefollowing specific but non-limiting examples. The following examples mayinclude compilations of data that are representative of data gathered atvarious times during the course of development and experimentationrelated to the presently-disclosed subject matter.

EXAMPLES Example 1

This Example demonstrates a novel strategy in DSC literature, where asingle chromophore can be used to increase the voltage of a tandem DSCsystem through careful photon management based on photoactive electrodepreparation methods. Through: (1) judicious control of electrode filmthickness, (2) proper dye selection, (3) careful redox shuttleselection, and (4) addition of a film surface protection technique,sequential ST-DSCs or SSM-DSCs (mechanically stacked with a singleilluminated area) can reach record high V_(oc) values. Intriguingly, thephotovoltage is obtained with only the use of early photons (400-700 nm)and metal-free organic sensitizers D35 and Y123 (FIGS. 1B-D) along withlow energy redox shuttles [Co(bpy-pz)₂]^(3+/2+) (FIG. 1E) and[Co(bpy)₃]^(3+/2+) (FIG. 1F).

Physically stacked and electrically connected ST-DSCs were finely tunedfor each device TiO₂ thickness and number of individual devices toachieve high V_(oc) values. Optimization of electrolyte additive(tert-butylpridine, TBP) concentration and surface capping withfluorinated self-assembled mono layer (FSAM), such asperfluorooctyltrimethylsilane (PFTS) (FIG. 1G), for individual devicesplayed a key role in achieving high V_(oc) and PCE mainly for front D35devices. This was validated by small modulation photovoltage transientmeasurements showing decreased electron recombinations with theelectrolyte.

Methods and Materials

All commercially obtained reagents and extra dry solvents were used asreceived. D35 and Y123 were purchased from Dyenamo, Sweden. ¹H NMRspectra were recorded on a Bruker Avance-500 (500 MHz) spectrometer andare reported in ppm using solvent as an internal standard (CDCl₃ at 7.26ppm). Data reported as s=singlet, d=doublet, t=triplet, q=quartet,p=pentet, m=multiplet, br=broad, ap=apparent, dd=doublet of doublets,and coupling constant(s) are in Hz. The optical and electrochemicalproperties of Y123, D35, and the Ru-sensitizer HD-2-mono (FIG. 1H) areshown in Table 1 and FIG. 1I.

TABLE 1 Optical and electrochemical properties of D35, Y123, andHD-2-mono. λ_(onset) λ_(max) ε E(_(S+/S)) E(_(S+/S*)) E_(g) ^(opt) Dye(nm)^(a) (nm)^(a) (M⁻¹cm⁻¹)^(a) (V vs NHE)^(b) (V vs NHE)^(c) (eV)^(d)D35 570 500 31000 1.10 −1.08 2.18 Y123 650 540 49000 1.07 −0.94 2.01HD-2-mono 680 539 13900 0.81 −1.02 1.83 ^(a)D35 was measured in DCM.⁹Y123 was also measured in DCM.⁷ HD-2-mono was measured in DMF.¹⁰λ_(onset) was estimated by drawing a tangent line on the low energy sideof the lowest energy transition and reporting the value as theintersection of the x-axis. ^(b)D35 and Y123 were measured on TiO₂.¹¹HD-2-mono was measured in DMF with glassy carbon working electrode,platinum wire as counter electrode and Ag/AgNO₃ as the referenceelectrode. Ferrocene was used an internal reference to calibrate thereference electrode. Ferrocene was taken as 0.63 V vs NHE in DMF.^(c)E(_(S+/S*)) = E(_(S+/S)) − E_(g) ^(opt). ^(d)E_(g) ^(opt) wascalculated from the equation E_(g) ^(opt) = 1240 (nm)(eV)/ λ_(onset).

Synthesis of Co(bpy-pz)₂ ^(+2/+3). A modified method of synthesizing abpy-pz ligand for the Co(bpy-pz)₂ ^(+2/+3) complexes is illustrated inFIG. 2A and described below. Characterization data matches that of theknown compounds.

6-fluoro-2,2′-bipyridine

To an oven dried, N₂ filled round bottom flask was added 17 ml of a 0.5M solution of pyridin-2-ylzinc (II) bromide (8.50 mmol) in THE and2-bromo-6-fluoropyridine (1.0 g, 5.67 mmol). Then Pd(PPh₃)₄ (0.131 g,0.11 mmol) was added as a single portion at room temperature. Thereaction mixture was stirred at room temperature overnight. The reactionwas monitored by TLC until the starting bromo-pyridine had beenconsumed. The crude product was poured into an EDTA/K₂CO₃ aqueoussolution (˜100 mL saturated in both EDTA and K₂CO₃) and extracted withEt₂O (˜100 mL). The organic layer was separated and dried with Na₂SO₄.After concentration, the product was purified by silica gel (125 mLSiO₂) chromatography with a gradient elution starting with 10% andprogressing to 20% ethyl acetate/hexanes to give a white solid (563 mg,3.23 mmol, 57% yield). ¹H NMR (300 MHz, d₆-DMSO) δ 8.71 (d, J=4.7 Hz,1H), 8.32 (dd, J=7.4, 2.7 Hz, 1H), 8.26 (d, J=7.9 Hz, 1H), 8.15 (ap q,J=7.7 Hz, 1H), 7.96 (dt, J=7.7, 1.6 Hz, 1H), 7.50 (dd, J=7.2, 4.7 Hz,1H), 7.21 (dd, J=8.1, 2.8 Hz, 1H). This molecule was previously preparedthrough an alternate procedure and the characterization data was inagreement.

6-(1H-pyrazol-1-yl)-2,2′-bipyridine

To an oven dried, N₂ filled round bottom flask was added 1H-pyrazole(479 mg, 7.04 mmol), sodium tert-butoxide (678 mg, 7.06 mmol), and 5 mlof N₂-degassed, dry DMSO at room temperature. Note: reaction gives offheat. After stirring for 10 minutes the reaction ceased to give offheated. 6-fluoro-2,2′-bipyridine (1.2 g, 6.89 mmol) was added in oneportion. The reaction was heated to 100° C. with stirring and monitoredby TLC. After 24 hours, the reaction was cooled to room temperature andthe mixture was extracted with Et₂O (˜100 mL) and H₂O (˜100 mL). Theorganics were separated and dried with Na₂SO₄. The concentrated mixturewas then purified by silica gel chromatography first by eluting 100% DCMto remove any excess pyrazole, then 50% MeOH/DCM to give a white solid(1.3 g, 5.99 mmol, 87% yield). ¹H NMR (300 MHz, d₆-DMSO) δ 8.92 (d,J=1.8 Hz, 1H), 8.72 (d, J=3.8 Hz, 1H), 8.58 (dd, J=7.7, 0.8 Hz, 1H) 8.33(d, J=7.7 Hz, 1H), 8.11 (t, J=7.7 Hz, 1H), 8.05-7.95 (m, 2H), 7.87 (d,J=0.7 Hz, 1H), 7.51 (dd, J=7.7, 4.8 Hz, 1H), 6.64 (ap t, J=1.6 Hz, 1H).This molecule was previously prepared through an alternate procedure andthe characterization data was in agreement.

Further complexation and oxidation, along with preparation Co(bpy-pz)₂^(+2/+3) using the bpy-pz ligand is illustrated in FIG. 2B.

Photovoltaic Characterization. Photovoltaic characteristics weremeasured using a 150 W xenon lamp (Model SF300A, SCIENCETECH Inc. ClassAAA) solar simulator equipped with an AM 1.5 G filter for a less than 2%spectral mismatch. Prior to each measurement, the solar simulator outputwas calibrated with a KG5 filtered mono-crystalline silicon NRELcalibrated reference cell from ABET Technologies (Model 15150-KG5). Thecurrent density-voltage characteristic of each cell was obtained withKeithley digital source meter (Model 2400). The incidentphoton-to-current conversion efficiency was measured with an IPCEinstrument manufactured by Dyenamo comprised of a 175 W xenon lamp(CERMAX, Model LX175F), monochromator (Spectral Products, Model CM110,Czerny-Turner, dual-grating), filter wheel (Spectral Products, ModelAB301T, fitted with filter AB3044 [440 nm high pass] and filter AB3051[510 nm high pass]), a calibrated UV-enhanced silicon photodiodereference and Dyenamo issued software.

Electron lifetime measurements through small modulation photovoltagetransient measurements, were carried out with a Dyenamo Toolbox(DN-AE01) instrument and software. The intensity of the LED light source(Seoul Semiconductors, Natural White, S42182H, 450 nm to 750 nmemission) is varied to modulate the device open-circuit voltage. Thebase light intensity was modulated by applied voltages of 2.80, 2.85,2.90, 2.95 and 3.00 V applied to the LED with the 3.0 V bias approaching1 sun intensity (97%). The direction of illumination was from thephotoanode to the counter electrode, and the device was positioned 5 cmfrom the LED light source. The voltage rise and decay times are fittedwith a Levenberg-Marquardt fitting algorithm via LabView, and theelectron lifetime was obtained from the averaging of rise and decaytimes.

Charge extraction at open circuit conditions (Q_(oc)) as function oflight intensity was carried out with a Dyenamo Toolbox (DN-AE01)instrument and software. Different open-circuit values were achieved bythe programmed control of a biased LED (description above) from 2.5 V to3.2 V. The LED is switched on for 1 second of illumination, thenswitched off for 10 seconds with a simultaneous switch to short-circuitconditions and monitoring of current. The total charge is found byintegrating the current measured over time.

Photoanode preparation: TEC 10 FTO glass (10 Ω/sq. sheet resistance: FTO[fluorine doped tin oxide]) for the final subcell in the SSM-DSC deviceswith scattering layer and TEC 15 FTO glass (15 Ω/sq. sheet resistance:FTO) for all other subcells was purchased from Hartford Glass. Once cutinto 2×2 cm squares the substrate was submerged in a 0.2% Deconex 21aqueous solution and sonicated for 15 minutes at room temperature. Theelectrodes were rinsed with water, sonicated in acetone 10 minutes, andsonicated in ethanol for 10 minutes. The electrodes were then placedunder UV/ozone for 15 minutes (UV-Ozone Cleaning System, ModelProCleaner by UVFAB Systems). A compact TiO₂ underlayer was then appliedby pre-treatment of the substrate submerged in a 40 mM TiCl₄ solution inwater (prepared by careful addition of 99.9% TiCl₄ to water at 0-5° C.).The submerged substrates (conductive side up) were heated for 30 minutesat 70° C. After heating, the substrates were rinsed first with waterthen with ethanol.

A thickness between 1.2 and 4.5 m mesoporous P30 TiO₂ layer (particlesize: 30 nm, Dyenamo, DN-GPS-30TS, >99% anatase) was screen printed froma Sefar screen (54/137-64W) resulting in 4.5 m or Sefar screen(90/230-48W) resulting in 2.2 m thickness on average for each print. Forthickness of 1.2 m and 0.8 m, the commercial P30 paste was diluted withterpineol (2:1, paste:terpineol) and (1:1, paste:terpineol),respectively, and printed with Sefar screen (90/230-48W). For thicknessof 1.5 m, P30 paste was diluted with terpineol (2:1, paste:terpineol)and printed with Sefar screen (54/137-64W). Particle sizes wereindicated to be typically within +2 nm of the average and are >99%anatase at the time of paste formulation as indicated by thedistributor. A 4.5 m TiO₂ scattering layer (for full device) (particlesize: >100 nm, Solaronix R/SP) was screen printed onto electrodes whereindicated.

Between each print of TiO₂ paste, the substrate was heated for 7 minutesat 125° C. and the thickness was measured with a profilometer(Alpha-Step D-500 KLA Tencor). The substrate was then sintered withprogressive heating from 125° C. (5 minute ramp from r.t., 5 minutehold) to 325° C. (15 minute ramp from 125° C., 5 minute hold) to 375° C.(5 minute ramp from 325° C., 5 minute hold) to 450° C. (5 minute rampfrom 375° C., 15 minute hold) to 500° C. (5 minute ramp from 450° C., 15minute hold) using a programmable furnace (Vulcan® 3-Series model3-550). The cooled, sintered photoanode was soaked 30 minutes at 70° C.in a 40 mM TiCl₄ water solution and heated at 40° C. per minute to 500°C. and held at 500° C. for 30 minutes prior to sensitization. Thecomplete working electrode was prepared by immersing the TiO₂ film intoa D35 dye solution (0.3 mM, 4:1 (EtOH:THF)), with 40× (12 mM) ofchenodeoxycholic acid (CDCA), or Y123 dye solution (0.2 mM, 1:1(ACN:tert-butanol)) with 50×CDCA. The electrodes were immersed in thedye solutions at room temperature for 6 hours.

Counter Electrode Preparation: Two holes were drilled through theinsulating side to the conductive side of 2×2 cm squares of TEC 7 FTOglass (7 Ω/sq. sheet resistance) for full subcells, and TEC 15 FTO glass(15 Ω/sq. sheet resistance) for all other subcells, using a Dremel-4000with a Dremel 7134 Diamond Taper Point Bit submerged in water to reduceglass cracking with Scotch tape on the FTO side to minimize scratching.The tape was removed, and the electrodes were washed with water followedby a 0.1 M HCl in EtOH solution. The electrodes were then sonicated inan acetone bath for 10 minutes and dried at 400° C. for 15 minutes. Forplatinum electrodes: A thin layer of Pt-paste (Solaronix, Platisol T/SP)was slot printed with a punched Scotch tape piece on the conductiveside. The electrodes were then heated at 450° C. for 10 minutes.

PEDOT electrodes: PEDOT electrodes were prepared following thepreviously reported procedure. Briefly, an aqueous solution of 0.1 Msodium dodecylsulfate (SDS, Biotechnology Grade, VWR) and 0.01 M3,4-ethylenedioxythiophene (EDOT, 97%, Alfa Aesar) was prepared throughsonicating for 90 minutes. Electro-oxidative polymerization wasperformed by a CHI 6054E potentiostat. In a two electrode cell, a 2×2 cmcleaned, predrilled FTO glass substrate was used as a working electrodeand same size cleaned FTO was used as counter and reference electrode.Electro-oxidative polymerization was performed under galvanostatic(chronopotentiometry on the instrument) mode with the followingparameters: anodic current valued at 3 mA, high and low potential (E)3.5-2.5 V, low E hold time 0 seconds, cathodic time of 0.005 seconds,anodic time 70 seconds, with initial polarity anodic, data storageinterval as 0.1 seconds and number of segments 2, where currentswitching priority was based on time. The blue colored films were rinsedthoroughly with deionized water and dried for 2 minutes at 125° C.before use.

Dye-sensitized Solar Cell assembly: The photoanode and counter electrodewere sealed with a 25 m thick hot melt gasket (Surlyn, Solaronix,“Meltonix 1170-25”) by heating the system at 130° C. under a pressure of0.2 psi for 1 minute with a sealing machine (Dyenamo, product DN-HM01).The electrolyte was added through the pre-drilled holes in the counterelectrodes with the rubber sealing tip from a Solaronix “Vac'n'FillSyringe” until the electrolyte began to emerge from the second counterelectrode hole. The holes were sealed with a Surlyn sheet and a thinglass cover by heating at 130° C. under pressure (0.1 psi) for 25seconds both for full and thin devices. Finally, soldered contacts wereadded with a MBR Ultrasonic soldering machine (model USS-9210) withsolder alloy (Cerasolzer wire diameter 1.6 mm, item #CS186-150). Acircular black mask (active area 0.15 cm²) punched from black tape wasused in the subsequent photovoltaic studies.

Photoanode Surface Treatment. Sensitized TiO₂ films were submerged in a0.1 M solution of 97% 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (PFTS)in hexanes for 90 minutes at 30° C. The electrodes were rinsed withhexanes and assembled as described above.

Preparation of CO₂ reduction Electrodes (Au₂O₃ and IrO₂). Gold oxide(Au₂O₃) and Iridium oxide (IrO₂) electrodes were prepared according toliterature. The ratio of the area of Au₂O₃ to IrO₂ electrode was(1:4.5). Briefly, for the preparation Au₂O₃ electrode, gold foil (99.95%metal basis, 0.1 mm thickness, 25*25 mm, Alfa Aesar, cut into 0.8 cm²(L*W, 0.4 cm*1 cm) was rinsed with diluted aqua regia mixed withdeionized water (1:3), which was subsequently oxidized in 0.5 M H₂SO₄ byapplying step functions, with a CHI 6054E potentiostat, by applyingsquare pulses between 1.183 and 3.183 V versus Ag/AgCl (KCl sat.) with alarge area platinum foil counter electrode at 500 Hz (2 ms period) with1,800,000 million cycles in a single chamber cell. Note: Extending thenumber of cycles to 3,600,000 led to lower performance of the Au₂O₃electrode and a powder to be observed to fall off the electrodes duringlonger cycle times. Images of the electrodes over time has been addedfor the 1,800,000 cycle case to show the change in electrode color overtime which was observed for high performing electrodes (FIG. 3 ).

The Au₂O₃ cathode was further rinsed with water carefully and dipped in0.5 M NaHCO₃ solution for 2-3 minutes before use in the electrochemicalcell. Briefly, for the anode preparation, a titanium foil (99.7%, 0.25mm thick [3 cm*1.5 cm], Sigma Aldrich) was etched for 60 minutes inboiling 1.0 M oxalic acid, then a 0.2 M solution of H₂IrCl₆ (CAS#110802-84-1, 99.9% Ir, STREM Chemicals) in isopropanol was drop castonto the electrode in order to cover the entire surface. The titaniumfoil was first dried at 70° C. for 10 minutes in air followed bycalcination at 500° C. for 10 minutes in the same ovens used for thepreparation of TiO₂ electrodes for DSCs (Vulcan® 3-Series model 3-550).This process was repeated three times on each side of the titanium foil.The prepared electrodes were further tested with cyclic voltammetry forcurrent-voltage properties with a saturated CO₂ in 0.5 M NaHCO₃ aqueoussolution. The required size 3.6 cm² (3 cm*0.6 cm) was then cut from theprepared IrO₂ electrodes for use in the electrochemical cells.

Electrochemical Cell (EC) and Measurements. Electrolysis was performedin a three neck flask with total volume of 61 ml. Au₂O₃ (area 0.8 cm²)and IrO₂ (area 3.6 cm²) electrodes were completely dipped in 25 ml of a0.5 M NaHCO₃ aqueous solution along with a Ag/AgCl (CH Instruments)reference electrode sealed by septa (Suba Seal, Sigma Aldrich). CO₂ wasbubbled through the solution for 30 minutes with an exit needle in placebefore sealing the system. The CO₂ saturated system was then activatedat −0.65 V vs Ag/AgCl until a steady current was reached (˜500 seconds).The CO produced during this period (and the amount of CO/currentgenerated in the first 20 minutes of SSM-DSC powered catalysis) wassubtracted from the reported CO values to ensure all catalysis waspowered by the SSM-DSC device. CO, H₂ and O₂ were detected by a dualdetector (TCD for H₂/O₂, FID for CO/O₂ with a methanizer) custom Agilent7890B gas chromatograph instrument (additional details below).

After this the two electrodes, Au₂O₃ and IrO₂, were connected to aSSM-DSCs device (active area 0.5 cm²) illuminated by a solar simulator(AAA rated for AM 1.5G sunlight) at 100 mWcm⁻². The continuous currentflow through the photovoltaic-electrochemical (PV-EC) cell was monitoredby a potentiostat (CHI 6054E) under controlled potential electrolysis(current versus time) conditions with no applied potential. Duringelectrolysis headspace samples (300 μL) were taken at differentintervals with a VICI valved syringe. The gas in the syringe wascompressed to 250 μL, then with the tip of the syringe submerged in avial of ether, the valve was open to allow the pressure to equalize toatmospheric pressure. The entire 250 μL sample was then injected onto acustom Agilent 7890B Gas Chromatograph (column, Agilent PorapakQ 6 ft, ⅛OD) with a dual detector system (TCD and FID), a methanizer before theFID detector, and a backflush system to maintain good spectrum qualityin the presence of CO₂. Quantitation of CO and methane was made usingthe FID detector and H₂ was quantified on the TCD detector (allcalibrated using standards purchased from BuyCalGas.com).

Discussion

DSCs operate by photoexcitation of a dye, which then transfers anelectron from the excited-state to TiO₂. The electron then traverses anexternal circuit before being collected at a counter electrode andreturning to the oxidized dye. The overall power conversion efficiency(PCE) is calculated through the equation: PCE=(V_(oc)×J_(sc)×FF)/I_(o),where V_(oc) is the open circuit voltage, J_(sc) is the short-circuitcurrent density, FF is the fill factor, and I_(o) is the incident lightintensity. The redox shuttle should only be slightly higher in energythan the ground-state energy of the dye for minimal photovoltage loss.

Metal free organic sensitizers chosen for this study (D35 and Y123), arewidely studied and well known for high absorptivity, aggregation freeelectrode sensitization and efficient PCEs in DSCs along with Co^(3+/2+)redox shuttles. D35 offers characteristic orange color with absorptionλ_(max) at 500 nm and Y123 offer up to 40 nm red shifted lightabsorption properties (FIG. 4 ). Furthermore, redox shuttles ofCo(bpy-pz)₂ ^(3+/2+) and Co(bpy)₃ ^(3+/2+) are capable of a theoreticalmaximum V_(oc) of 1.36 V and 1.06 V, respectively, depending on thedifference between redox potential and the energy level difference ofTiO₂ with the conduction band energy of TiO₂ taken as −0.5 V versus NHE(FIG. 4 ). However, the highest reported values for Co(bpy-pz)^(3+/2+)are up to 300 mV and for Co(bpy)^(3+/2+) up to 160 mV lower than thetheoretically possible for either D35 and Y123. On the other hand, I⁻/I₃⁻ system is only capable of theoretical maximum V_(oc) of 0.9V, andV_(oc) high as 0.882 V was recently reported.

Under the conditions described herein (FIG. 5 , Table 2) with previouslyemployed electrolyte concentrations, PCE of up to 5.1% and V_(oc) of0.950 V was achieved for D35 with Co(bpy-pz)₂ ^(3+/2+). The V_(oc) andPCE were further improved to 0.998 V and 5.9%, respectively, byincreasing the concentration of Co(bpy-pz)₂ ^(3+/2+) from 0.22 M to 0.25M and the concentration of tert-butylpyridine (TBP) from 0.2 M to 0.5 M(FIG. 5 , Table 2). An insulating capping layer (PFTS:perfluorooctyltrimethylsilane) was then used to slow the recombinationof electrons in TiO₂ with the redox shuttles, which led to V_(oc) valuesof up to 1040 mV, with a PCE of 6.8% (Table 2 Entries 2 & 3). This isbelieved to be the first time a fluorinated self-assembled monolayer(FSAM) treatment such as PFTS has been achieved with a Co^(3+/2+) redoxsystems. The device was found to improve in PCE through an increase inboth V_(oc) and J_(sc). An increase in electron lifetime in TiO₂ (FIG. 6) was observed with a PFTS treatment, indicating slower recombinationkinetics. However, no shift in the conduction band of TiO₂ was observedbased on charge-extraction from open-circuit conditions as a function oflight intensity, which indicates that the gain in voltage is due toslowing recombination losses (FIG. 6 ).

Turning to Y123, V_(oc) of up to 0.960 V and PCE of up to 8.1% wereobtained, on average, with PEDOT as the counter electrode and withoutanti-reflecting coating (Table 2, entries 4-6). Unlike D35 devicesthough, Y123 based-devices were found to show lower performance as aresult of apparent dye desorption from the TiO₂ surface with PFTStreatment (Table 2). Accordingly, only D35 devices employ a PFTStreatment in the remaining studies.

TABLE 2 D35 and Y123 optimizations with Co(bpy-pz)₂ ^(3+/2+) electrolyteand FSAM surface treatment. Entry Dye and TiO₂ Thickness TreatmentV_(oc) (mV) J_(sc) (mA/cm²) FF PCE % Co(bpy-pz)₂ ^(3+/2+) electrolyteand surface treatment optimization with a scattering layer 1 D35 fullNone^(a) 953 ± 3 7.1 ± 0.3 74 ± 1 5.1 ± 0.3 2 None 998 ± 7 7.9 ± 0.2 72± 4 5.9 ± 0.4 3 FSAM 1037 ± 15 8.7 ± 0.4 74 ± 2  6.8 ± 0.08 4 Y123 fullNone 965 ± 0 9.7 ± 0.3   67 ± 0.2 6.6 ± 0.2 5 None^(b)  959 ± 21 11.4 ±0.3  72 ± 2 8.1 ± 0.3 6 FSAM^(b) 946 ± 5 9.6 ± 0.3   71 ± 0.9 6.5 ± 0.3Co(bpy-pz)₂ ^(3+/2+) devices with thin TiO₂ films, without a scatteringlayer 7 D35 0.8 μm FSAM 981 ± 7 2.8 ± 0.1 70 ± 3   2 ± 0.2 8 D35 1.2 μmFSAM  979 ± 17 3.7 ± 0.1   69 ± 0.5 2.5 ± 0.1 9 D35 1.5 μm FSAM 997 ± 95.1 ± 0.2 69 ± 2 3.5 ± 0.1 10 D35 2.2 μm FSAM 1031 ± 8  7.1 ± 0.5 68 ± 25.4 ± 0.4 11 Y123 1.2 μm None 1010 ± 2  5.2 ± 0.1   64 ± 1.4 3.5 ± 0.212 Y123 2.2 μm None 1010 ± 2   7.5 ± 0.05   70 ± 1.7  5.4 ± 0.08 Allcells employ an optimized electrolyte having concentrations as: 0.25MCo(bpy-pz)₂(PF₆)₂, 0.05M Co(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5Mtert-butylpyridine, in acetonitrile unless otherwise noted. Devicesemploy a Pt counter electrode unless otherwise noted. Device parametersare the average of 3 cells. Active area was fixed at 0.15 cm² throughoutthe study, with a black tape mask. Values after cell type refer to TiO₂film thickness, “full” refers to a 4.5 μm active layer of 30 nmparticles and a 4.5 μm scattering layer. ^(a)Electrolyte withconcentrations as previously reported for Y123, which was comprised of:0.22M Co(bpy-pz)₂(PF₆)₂, 0.05M Co(bpy-pz)₂(PF₆)₃, 0.1M LiClO₄, and 0.2Mtert-butylpyridine, in acetonitrile.⁵ ^(b)PEDOT counter electrode used.

Careful control of device thicknesses for each electrode in the SSM-DSCsystem is used to balance the photon distribution to each subcell bystrategically allowing light to pass through the first electrodes.Judicious photon management strategies for SSM-DSC devices are criticalfor high performances since subcells wired in series are limited by thelowest current subcell in the SSM-DSC device, and the maximal current isproportional to the number of photons absorbed by a subcell.Accordingly, for application of D35 and Y123, in mechanically stackedseries connected tandem devices, TiO₂ active layer thickness wasminimized without a scattering layer to permit sufficient light to passthrough to bottom cells. More specifically, in this Example, the TiO₂film active layer thickness for the front subcells was evaluated between0.8-2.2 μm without a scattering layer (FIG. 5 , Table 2 entries 7-10).The range of TiO₂ thickness was controlled by diluting the regular pastewith terpineol (paste: terpineol, 1:1 or 2:1) and printing screen forthinner films.

The IV and IPCE of the different thickness devices are shown in FIG. 5and Table 2. For devices without a scattering layer, s values rangingfrom 2.8-7.1 mA/cm² and IPCE peak values ranging from 38-75% could beobtained with D35 through film thickness control (FIG. 5 ). In a similarfashion, the J_(sc) of Y123 based devices could be modulated between 5.2and 7.5 mA/cm² by changing the film thickness (FIG. 5 , Table 2 entries11-12). For devices with a scattering layer, a peak IPCE value of 83% at490 nm was achieved for D35 and a peak IPCE value of 74% at 530 nm wasachieved for Y123. Increase in thickness from 0.8-2.2 μm showed linearincrease in IPCE response for both D35 and Y123.

With a method for tuning device photocurrents in hand, SSM-DSC deviceswith 2-5 subcells were constructed by judicious selection of subcellthicknesses (FIGS. 7A-C, Tables 3-4). Ideally, irradiation should beequally distributed among the subcells according to photon flux,segregated based on photon potential energy, and minimal transmittedphotons should be allowed through the SSM-DSC system. Thus, the frontsubcell was varied depending on the total number of subcells in theSSM-DSC to ensure maximal efficiency.

TABLE 3 SSM-DSC results for 2-3 subcell systems with D35, Y123 andHD-2-mono Entry Subcell Type Thickness (μm) V_(oc) (mV) J_(sc) (mA/cm²)FF PCE % 1 D35/D35 1.5/full 1965 ± 5  4.9 ± 0.1 0.71 ± 0.01 6.9 ± 0.2 2D35/Y123 1.5/full 1918 ± 19 5.9 ± 0.2 0.62 ± 0.01 7.1 ± 0.2 3D35/D35/Y123 1.2/2.2/full 2871 ± 14 3.9 ± 0.2 0.55 ± 0.04 6.4 ± 0.1 4D35/Y123/Y123 1.2/2.2/full 2809 ± 8  3.7 ± 0.1 0.61 ± 0.01 6.4 ± 0.1 5D35/Y123/Y123^(a) 1.2/2.2/full 2666 ± 5  3.6 ± 0.2 0.72 ± 0.02  7.0 ±0.02 6 D35/Y123/HD-2-mono 1.2/2.2/full 2628 ± 17 3.9 ± 0.3 0.74 ± 0.037.7 ± 0.1 Front cells are the first dyes listed with the respectivethickness of the TiO₂ film listed first as well. Subsequent subcells arelisted in order from front to back. “Full” indicates subcells use a 4.5μm TiO₂ active layer and a 4.5 μm TiO₂ scattering layer. Electrolyteused for all cells unless otherwise noted is as follows: 0.25MCo(bpy-pz)₂(PF₆)₂, 0.05M Co(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5Mtert-butylpyridine, in acetonitrile. ^(a)Indicates Co(bpy)₃ ^(3+/2+) wasused in this subcell

TABLE 4 SSM-DSC results for comparison of different front cellsthicknesses. Entry Subcell Type Thickness (μm) V_(oc) (mV) J_(sc)(mA/cm²) FF PCE % 1 D35/Y123 1.5/full 1918 ± 19 5.9 ± 0.2 0.62 ± 0.017.1 ± 0.2 2 D35/Y123 2.2/full 1930 ± 10 7.2 ± 0.2 0.51 ± 0.01 7.2 ± 0.33 D35/D35/Y123/Y123 0.8/2.2/2.2/full 3718 ± 8  2.7 ± 0.1 0.58 ± 0.03 5.8± 0.2 4 D35/D35/Y123/Y123 1.2/2.2/2.2/full 3776 ± 9  4.1 ± 0.3 0.35 ±0.02 5.4 ± 0.2 5 D35/D35/Y123/Y123/Y123 0.8/2.2/1.2/2.2/full 4670 ± 382.4 ± 0.1 0.38 ± 0.05 4.3 ± 0.3 6 D35/D35/Y123/Y123/Y1231.2/2.2/1.2/2.2/full 4667 ± 32 3.7 ± 0.1 0.22 ± 0.03 3.8 ± 0.4 Frontcells are the first dyes listed with the respective thickness of theTiO₂ film listed first as well. Subsequent subcells are listed in orderfrom front to back. “Full” indicates subcells use a 4.5 μm TiO₂ activelayer and a 4.5 μm TiO₂ scattering layer. Electrolyte used for all cellsunless otherwise noted is as follows: 0.25M Co(bpy-pz)₂(PF₆)₂, 0.05MCo(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5M tert-butylpyridine, inacetonitrile.

For a two subcell SSM-DSC with D35/D35 (1.5 μm/full), a record highV_(oc) is obtained at 1.97 V with a PCE of 6.9% (FIGS. 7A-B, Table 3entry 1). A D35/Y123 (1.5 μm/full) SSM-DSC using a broader absorbingsecond dye led to a Vo, of 1.92 V and higher PCE of 7.1% (FIGS. 7A-B,Table 3 entry 2). The substantially improved efficient light harvestingin the tandem cells with D35 at the bottom as compared to Y123 at thebottom is believed to be due to better complement of top and bottom celllight absorption properties. A loss of FF was also observed for theD35/Y123 SSM-DSC when compared to the D35/D35 system (0.71 vs 0.62).This loss is due to a mismatch of current through the two devices. Itwas further observed that thickness of the front device showed a largeeffect on FF and J_(sc) whereas overall PCE was comparable (Table 4entries 1-2).

Adding a third subcell (D35/D35/Y123, 1.2/2.2/full) further increasedthe device photovoltage to 2.87 V with a PCE of 6.4%. Changing thecenter subcell to Y123 led to a comparable V_(oc) of 2.81 V with a PCEof 6.4%, however; a significant increase in FF was observed for theD35/Y123/Y123 system at 0.61 (from 0.55) due to a better matching ofsubcell J_(sc) values. Although the current is generally thought to belimited by the minimum current device in series tandem devices, acurrent pushing-up (i.e., tandem device current higher than the lowestcurrent device) was noticed. In this configuration, the third cell wasfound to be current limiting via IV and IPCE measurements anddiminishing overall device performances (FIGS. 8A-9B, Table 3 entries4-6). FIG. 8C compares the IPCE for subcells in an SSM-DSC device with acombined IPCE of three subcells showing a peak IPCE of ˜60% near 600 nm.Changing the redox shuttle system from [Co(bpy-pz)₂]^(2+/3+) to[Co(bpy)₃]^(2+/3+) for the third cell alone (Table 3 entry 5) gave asignificant increase in current leading to an SSM-DSC with a V_(oc) 2.67V and 7.0% PCE. To further increase the current at the back cell, abroader absorbing Ru-sensitizer was selected (HD-2-mono) with I⁻/I₃ ⁻ asthe redox shuttle system (FIG. 2 ). A D35/Y123/HD-2-mono SSM-DSC systemgives a high PCE of 7.7% with a Vo, of 2.62 V. Both the D35/Y123/Y123and D35/Y123/HD-2-mono devices gave excellent fill factors (0.72 and0.74, respectively) owing to very well balanced currents through allthree devices as shown by change in IV and IPCE curves (FIGS. 8A-9B).

The device stacks were also analyzed with different front devicethicknesses, where the thickness of each individual device was chosenstrategically to allow enough light to pass to next device. Thinnerfront devices including TiO₂ with a thickness of 0.8 μm lead to overallhigher PCE and FF. The effect of mismatched currents in differentdevices is illustrated in FIGS. 7A-C (middle right) by sudden humps. Itwas observed that instead of following the normal trend of current beinglimited by the lowest current device in the series tandem, J_(sc) and FFwere found to interchange. Referring to Table 3, device parametersgenerally followed the front device current, however, as noticed fromthe IV graphs and device parameters, high J_(sc) lead to lower FF andvice versa. To the best of our knowledge, this is a record demonstrationof photovoltage in a dye-sensitized solar cell system from a singleilluminated area.

To evaluate the equal distribution of photons, incidentphoton-to-current conversion efficiency (IPCE) (FIGS. 8A-E) and IVmeasurements (Tables 5-7) were made for each individual cell after beingconfigured in an SSM-DSC system. For a three device system with aCo(bpy-pz)₂ ^(3+/2+) redox shuttle, it was noted that in oneconfiguration the third device had a J_(sc) of 2.6 mA/cm², which wasdecreased as compared to the 3.7 mA/cm² J_(sc) of the front two devices(Table 6). However, increased J_(sc) was achieved by replacing the thirddevice with Y123 employing Co(bpy)₃ ^(3+/2+) and/or HD-2-mono employingan iodine redox shuttle (FIG. 9A, Table 6). With these configurations,J_(sc) for the third device in the stack increased to 3.2 and 3.6 mA/cm²for Y123 with Co(bpy)^(3+/2+) and HD-2-mono, respectively. As the resultof better current match in J_(sc) of three devices in the stack,increased FF (61% to 74%) and overall PCE (6.4% to 7.7%) was achieved aswell (FIG. 9B).

TABLE 5 Tandem device and its individual subcell photovoltaiccharacteristics for Co(bpy-pz)₂ ^(3+/2+) based electrolyte with two andthree stacked devices. Dye and TiO₂ Thickness V_(oc) J_(sc) (μm)Position (mV) (mA/cm²) FF % PCE % D35/D35 D35 1.5 μm itself 997 ± 9 5.1± 0.2 69 ± 2 3.5 ± 0.1 D35 full itself 1037 ± 15 8.7 ± 0.4 74 ± 2  6.8 ±0.08 2nd 984 ± 8 4.9 ± 0.1 73 ± 2 4.0 ± 0.2 D35/D35 Tandem 1965 ± 5  4.9 ± 0.05 71 ± 1 6.9 ± 0.2 (1.5/full) D35/Y123 D35 1.5 μm itself 997 ±9 5.1 ± 0.2 69 ± 2 3.5 ± 0.1 Y123 full itself  959 ± 21 11.4 ± 0.3  72 ±2 8.1 ± 0.3 2nd  905 ± 13 6.2 ± 0.4 77 ± 2 4.5 ± 0.4 D35/Y123 Tandem1918 ± 19 5.9 ± 0.2 62 ± 1 7.1 ± 0.2 (1.5/full) D35/D35/Y123 D35 1.2 μmitself  979 ± 17 3.7 ± 0.1   69 ± 0.5 2.5 ± 0.1 D35 2.2 μm itself 1031 ±8  7.1 ± 0.5 68 ± 2 5.4 ± 0.4 2nd 987 ± 5 2.3 ± 0.2 71 ± 5  1.8 ± 0.03Y123 full itself  959 ± 21 11.4 ± 0.3  72 ± 2 8.1 ± 0.3 3rd  876 ± 263.3 ± 0.1 78 ± 1  2.4 ± 0.02 D35/D35/Y123 Tandem 2871 ± 14 3.9 ± 0.2 55± 4 6.4 ± 0.1 (1.2/2.2/full) “itself” refers to measurements made onthis subcell when taken out of the SSM-DSC configuration and measureddirectly. “2^(nd) or 3^(rd)” refers to measurements made on thisspecific subcell only, when in the SSM-DSC configuration. Front cellsare the first dyes listed with the respective thickness of the TiO₂ filmlisted first as well. Subsequent subcells are listed in order from frontto back. “Full” indicates subcells use a 4.5 μm TiO₂ active layer and a4.5 μm TiO₂ scattering layer. Electrolyte used for all cells unlessotherwise noted is as follows: 0.25M Co(bpy-pz)₂(PF₆)₂, 0.05MCo(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5M teri-butylpyridine, inacetonitrile.

TABLE 6 Tandem SSM-DSC devices and subcell photovoltaic characteristicsfor varying electrolyte at the final cell. Dye and TiO₂ V_(oc) J_(sc)Thickness Position (mV) (mA/cm²) FF PCE % front two subcells based onD35/Y123 used for the remainder of the table D35 1.2 μm itself  979 ± 173.7 ± 0.1  69 ± 0.5 2.5 ± 0.1 Y123 2.2 μm itself  998 ± 12 7.9 ± 0.4 71± 2  5.7 ± 0.3 2nd 958 ± 8 3.7 ± 0.2 76 ± 2  2.7 ± 0.2 3^(rd) (andfinal) subcell based on Y123 employing Co(bpy-pz)₂ ^(3+/2+) Y123 fullitself  959 ± 21 11.4 ± 0.3  72 ± 2  8.1 ± 0.3 3rd  883 ± 15  2.6 ± 0.0578 ± 1   1.9 ± 0.05 D35/Y123/ Tandem 2809 ± 8  3.7 ± 0.1 61 ± 1  6.4 ±0.1 Y123 3^(rd) (and final) subcell based on Y123 employing Co(bpy)₃^(3+/2+) Y123 full itself  811 ± 18 12.3 ± 0.5  73 ± 2  7.6 ± 0.2 3rd 732 ± 20 3.2 ± 0.1  78 ± 0.6 2.0 ± 0.1 D35/Y123/ Tandem 2666 ± 5  3.6 ±0.2 72 ± 2   7.0 ± 0.02 Y123 3^(rd) (and final) subcell based onHD-2-mono employing (I⁻/I₃ ⁻) HD-2-mono itself 719 ± 8 16.6 ± 0.1  0.73± 0.01 8.8 ± 0.1 full 3rd  688 ± 20  3.6 ± 0.05 78 ± 1  1.84 ± 0.1 D35/Y123/ Tandem 2628 ± 17 3.9 ± 0.3 74 ± 3  7.7 ± 0.1 HD-2-mono“itself” refers to measurements made on this subcell when taken out ofthe SSM-DSC configuration and measured directly. “2^(nd) or 3^(rd)”refers to measurements made on this specific subcell only, when in theSSM-DSC configuration. Front cells are the first dyes listed with therespective thickness of the TiO₂ film listed first as well. Subsequentsubcells are listed in order from front to back. “Full” indicatessubcells use a 4.5 μm TiO₂ active layer and a 4.5 μm TiO₂ scatteringlayer. Electrolyte used for all cells unless otherwise noted is asfollows: for Co(bpy-pz)₂ ^(3+/2+): 0.25M Co(bpy-pz)₂(PF₆)₂, 0.05MCo(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5M tert-butylpyridine, inacetonitrile; for Co(bpy)₂ ^(3+/2+): 0.25M Co(bpy-pz)₂(PF₆)₂, 0.05MCo(bpy-pz)₂(PF₆)₃, 0.1M LiTFSI, and 0.5M tert-butylpyridine, inacetonitrile; for I⁻/I₃ ⁻: electrode thickness was 10.0 μm 30 nm TiO₂particles and 4.5 μm scattering layer. Iodine electrolyte was used: 1.0MDMII (1,3-dimethylimidazolium iodide), 0.05M LiI, 30 mM I₂, 0.5M TBP(4-tert-butylpyridine), 0.1M GNCS (guanadinium thiocyanate) inacetonitrile and valeronitrile (v/v, 85/15).

TABLE 7 Tandem device and its individual subcell photovoltaiccharacteristics for Co(bpy-pz)₂ ^(3+/2+) based electrolyte with 4 and 5stacked devices. See Table 6 for device parameters. Dye and TiO₂ V_(oc)J_(sc) Thickness Position (mV) (mA/cm²) FF PCE % D35 (0.8μm)/D35/Y123/Y123 D35 0.8 μm front 981 ± 7 2.8 ± 0.1 70 ± 3    2 ± 0.2D35 2.2 μm itself 1031 ± 8  7.1 ± 0.5 68 ± 2  5.4 ± 0.4 2nd  970 ± 222.5 ± 0.3 71 ± 02 1.8 ± 0.1 Y123 1.2 μm itself  998 ± 12 7.9 ± 0.4 71 ±2  5.7 ± 0.3 3rd  930 ± 24 2.1 ± 0.3 77 ± 1  1.6 ± 0.3 Y123 full itself 959 ± 21 11.4 ± 0.3  72 ± 2  8.1 ± 0.3 4th  887 ± 14 1.9 ± 0.1 78 ± 3  1.4 ± 0.05 D35/D35/Y123/ Tandem 3718 ± 8  2.6 ± 0.1 58 ± 3  5.8 ± 0.2Y123 D35 (1.2 μm)/D35/Y123/Y123 D35 1.2 μm front  979 ± 17 3.7 ± 0.1  69± 0.5 2.5 ± 0.1 D35 2.2 μm itself 1031 ± 8  7.1 ± 0.5 68 ± 2  5.4 ± 0.42nd 987 ± 5 2.3 ± 0.2 71 ± 5   1.8 ± 0.03 Y123 1.2 μm itself  998 ± 127.9 ± 0.4 71 ± 2  5.7 ± 0.3 3rd 937 ± 5 1.7 ± 0.1 78 ± 1  1.3 ± 0.1 Y123full itself  959 ± 21 11.4 ± 0.3  72 ± 2  8.1 ± 0.3 4th 867 ± 5  1.6 ±0.05 79 ± 2   1.2 ± 0.02 D35/D35/Y123/ Tandem 3776 ± 10 3.9 ± 0.3 35 ±2  5.4 ± 0.2 Y123

IV and IPCE for D35, Y123, and HD-2-mono were also measured withCo(bpy)₃ ³⁺²³⁺ electrolyte (FIG. 10 , Table 8). As demonstrated in Table8, FSAM treatment with Co(bpy)^(3+/2+) and D35 increased V_(oc) by up to70 mV, similar to previously studied Ru (II) dye and I⁻/I₃ ⁻, whichimproved V_(oc) by up to 70 mV with similar TiO₂ films. The results fromCo(bpy)₃ ^(2+/3+) based SSM-DSC devices are shown in FIG. 11 and Tables9-10. Comparatively, Co(bpy-pz)₂ ^(3+/2+) based D35/D35 devices resultedin higher V_(oc) (1.965 V) compared to (1.861 V) of Co(bpy)₃ ^(3+/2+)due to inherently more positive redox potential of former, withcomparable overall PCE.

TABLE 8 D35, Y123 and HD-2-mono optimization with Co(bpy)₃ ^(3+/2+)electrolyte and FSAM surface treatment. Entry Dye and TiO₂ ThicknessTreatment V_(oc) (mV) J_(sc) (mA/cm²) FF PCE % Co(bpy)₃ ^(3+/2+)electrolyte and surface treatment optimization with scattering layer 1D35^(a,b) None 854 ± 6   10.3 ± 0.31   73 ± 1.8 6.6 ± 0.1 2 D35^(b) None854 ± 14 10.7 ± 0.5 74 ± 1 6.9 ± 0.2 3 FSAM 921 ± 15 10.2 ± 0.6 73 ± 4  7 ± 0.4 4 Y123^(b) none 842 ± 23 12.3 ± 1.3 72 ± 1 7.7 ± 0.4 5HD-2-mono^(c,d) none 719 ± 8  16.6 ± 0.1   73 ± 0.01 8.8 ± 0.1 Co(bpy)₃^(3+/2+) devices with different thickness and without scattering layer 6D35 1.2 μm FSAM 993 ± 23   3.2 ± 0.15   73 ± 1.4  2.4 ± 0.08 7 D35 2.2μm FSAM 974 ± 14  6.1 ± 0.4 67 ± 3 4.1 ± 0.2 8 Y123 1.2 μm None 973 ± 28 5.5 ± 0.3 66 ± 8 3.6 ± 0.3 9 Y123 2.2 μm None 898 ± 25  7.4 ± 0.96 70 ±4   5 ± 0.5 All cells employ the optimized electrolyte havingconcentrations as 0.25M Co(bpy)₃(PF₆)₂, 0.05M Co(bpy)₃(PF₆)₃, 0.1MLiTFSI, and 0.5M tert-butylpyridine in acetonitrile unless otherwisenoted. ^(a)Electrolyte with concentrations as: 0.25M Co(bpy)₃(PF₆)₂,0.05M Co(bpy)₃(PF₆)₃, 0.1M LiTFSI, and 0.25M tert-butylpyridine inacetonitrile. ^(b)Electrode thickness was 4.5 μm 30 nm TiO₂ particlesand 4.5 μm scattering layer. ^(c)Electrode thickness was 10.0 μm 30 nmTiO₂ particles and 4.5 μm scattering layer. ^(d)Iodine electrolyte wasused: 1.0M DMII (1,3- dimethylimidazolium iodide), 0.05M LiI, 30 mM I₂,0.5M TBP (4-tert-butylpyridine), 0.1M GNCS (guanadinium thiocyanate) inacetonitrile and valeronitrile (v/v, 85/15).

TABLE 9 Summary of Co(bpy)₃ ^(3+/2+) based SSM-DSCs. Entry Dye Thickness(μm) V_(oc) (mV) J_(sc) (mA/cm²) FF % PCE % 1 D35/D35 1.5/full 1824 ± 5 5.1 ± 0.1 71 ± 1 6.5 ± 0.1 2 D35/Y123 2.2/full 1832 ± 12 5.6 ± 0.2 75 ±3  7.6 ± 0.01 3 D35/D35/Y123 1.2/3/full 2666 ± 30 3.9 ± 0.1 52 ± 1 5.3 ±0.1 4 D35/Y123/Y123 2.2/2.2/full 2584 ± 30 3.4 ± 0.3 75 ± 5 6.7 ± 0.2Front cells are the first dyes listed with the respective thickness ofthe TiO₂ film listed first as well. Subsequent subcells are listed inorder from front to back. “Full” indicates subcells use a 4.5 μm TiO₂active layer and a 4.5 μm TiO₂ scattering layer. Electrolyte used forall cells unless otherwise noted is as follows: 0.25M Co(bpy)₃(PF₆)₂,0.05M Co(bpy)₃(PF₆)₃, 0.1M LiTFSI, and 0.5M tert-butyipyridine, inacetonitrile.

TABLE 10 Tandem device and its individual subcell photovoltaiccharacteristics for Co(bpy)₃ ^(+3/2+) based electrolyte with 2 to 5subcell SSM-DSC devices. Dye and TiO₂ V_(oc) J_(sc) Thickness Position(mV) (mA/cm²) FF PCE % D35/D35 D35 1.5 μm itself 937 ± 3 5.2 ± 0.2 66 ±2  3.3 ± 0.1  D35 full itself  921 ± 15 10.2 ± 0.6  73 ± 4   7 ± 0.4 2nd892 ± 7 5.0 ± 0.2 76 ± 1  3.6 ± 0.2  D35/D35 Tandem 1824 ± 5  5.1 ± 0.1 71 ± 1.1 6.5 ± 0.1  D35/Y123 D35 2.2 μm itself  974 ± 14 6.1 ± 0.4 67 ±3  4.1 ± 0.2  Y123 full itself  842 ± 23 12.3 ± 1.3  72 ± 1  7.7 ± 0.4 2nd  835 ± 33  5.6 ± 0.05 76 ± 2  3.5 ± 0.1  D35/Y123 Tandem 1832 ± 125.6 ± 0.2 75 ± 3  7.6 ± 0.01 D35/D35/Y123 D35 1.2 μm itself  987 ± 113.9 ± 0.1 69 ± 1  2.9 ± 0.03 D35 3 μm itself 948 ± 2   7 ± 0.4 73 ± 1 4.9 ± 0.2  2nd 930 ± 9 2.4 ± 0.3 71 ± 5  1.7 ± 0.1  Y123 foil itself 837 ± 23 11.9 ± 0.7  73 ± 2  7.6 ± 0.1  3rd 732 ± 2 3.9 ± 0.1  76 ± 0.52.4 ± 0.02 D35/D35/Y123 Tandem 2666 ± 30 3.9 ± 0.1 52 ± 1  5.3 ± 0.1 D35/Y123/Y123 D35 2.2 μm itself  974 ± 14 6.1 ± 0.4 67 ± 3  4.1 ± 0.2 Y123 2.2 μm itself  898 ± 25  7.4 ± 0.96 70 ± 4   5 ± 0.5 2nd 875 ± 53.1 ± 0.4 77 ± 4  2.2 ± 0.2  Y123 full itself  842 ± 23 12.3 ± 1.3  72 ±1  7.7 ± 0.4  3rd  732 ± 20 3.4 ± 0.1  78 ± 0.6 2.1 ± 0.1  D35/Y123/Y123Tandem 2584 ± 30 3.4 ± 0.3 75 ± 5  6.7 ± 0.2  “itself” refers tomeasurements made on this subcell when taken out of the SSM-DSCconfiguration and measured directly. “2^(nd), 3^(rd), 4^(th) or 5^(th)”refers to measurements made on this specific subcell only, when in theSSM-DSC configuration. Front cells are the first dyes listed with therespective thickness of the TiO₂ film listed first as well. Subsequentsubcells are listed in order from front to back. “Full” indicatessubcells use a 4.5 μm TiO₂ active layer and a 4.5 μm TiO₂ scatteringlayer. Electrolyte used for all cells unless otherwise noted is asfollows: 0.25M Co(bpy)₂(PF6)₂, 0.05M Co(bpy)₂(PF6)₃, 0.1M LiTFSI, and0.5M tert-butylpyridine, in acetonitrile.

Intriguingly, the SSM-DSC system maintains high photovoltages even underlow light conditions at 3.99 V at 10% sun intensity for a PCE of 7.0%(FIG. 12 , Table 11). This SSM-DSCs system is highly attractive forindoor light-to-electric applications requiring high voltages such as alithium ion battery charging at ˜3.7-4.0 V.

TABLE 11 Device parameters for a 5 subcell SSM-DSC device (the finalentry in Table 10) with the Co(bpy)₃ ^(3+/2+) redox shuttle measuredunder different % sun values. % Sun V_(oc) (mV) J_(sc) (mA/cm²) FF PCE %100% 4369 1.9 57.9 4.9  79% 4322 1.6 58 4.1 (5.2%)  50% 4256 1.1 58 2.8(5.6%)  32% 4190 0.8 59 2 (6.25%)  10% 3990 0.3 60 0.7 (7%)

Electrocatalytic Reduction of CO₂ to CO

An interesting approach for solar-to-fuel conversion is the applicationof integrated photovoltaic-electrochemical cell (PV-EC) devices thatoperate without external bias. As a demonstration of the utility of theSSM-DSC systems discussed above, a 3 subcell SSM-DSC system was used topower the electrocatalytic reduction of CO₂ to CO in aqueous media withno external bias using an IrO₂ anode and an Au₂O₃ cathode (FIG. 13 ).This represents the only DSC system driven with simulated sunlight andwater as the electron source for solar-to-CO conversion of CO₂ with noexternal bias. An overall efficiency for CO production (η_(co)) of 2%was calculated, with a solar-to-fuel efficiency of 2.7% when H₂ producedis accounted for using the equation:q_(co)=(J_(op)×E_((CO2/CO))×FE)/I_(o), where J_(op) is the operatingcurrent density, E_((CO2/CO)) is the thermodynamic free energy (1.43 Vfor this EC), FE is the Faradaic efficiency, and Jo is the incidentsolar power density (FIG. 14 ). The diminishment of J_(op) overtime wasfound to be due to deterioration of the performance of only the frontcell in the SSM-DSC system (FIG. 4 , Table 12). This loss in performanceover time for the front device was found to be fully circumventedthrough the use of a gel electrolyte along with a 385 nm UV cutofffilter (FIGS. 15-16 ).

TABLE 12 Comparison of SSM-DSC (D35/Y123/Y123 (Co(bpy)₃ ^(3+/2+)) deviceparameters before and after powering CO₂ reduction. Cell type V_(oc)(mV) J_(sc) (mA/cm²) FF PCE % SSM-DSC with 1.2 μm front device Beforeelectrolysis 2690 3.9 69 6.95 After electrolysis 2540 2.8 27 1.9 SSM-DSCwith 2.2 μm front device Before electrolysis 2695 3.7 60 6.2 Afterelectrolysis 2578 3.5 42 3.9

In a dramatic display of SSM-DSC device stability, a SSM-DSC device wasconstructed with an IrO₂ covered titanium electrode attached at theSSM-DSC cathode and a platinum wire connected to the SSM-DSC photoanode(FIG. 17 ). This device was fully submerged in an aqueous solution andirradiated with a solar simulator in a beaker open to air. A steady flowof O₂ and H₂ can be observed (FIG. 18 ) at the electrodes atapproximately 0.5 mL of gas per second using a laboratory test celldesign. This rate can be dramatically enhanced through increases in thesolar active area size. This stands in stark contrast to other solarcell technologies that require precautions such as Argon chambers.

Conclusion

Sequential series multijunction dye-sensitized solar cells (SSM-DSCs),which are mechanically stacked single illuminated area DSC devices wiredin series, are reported to have the highest photovoltages obtained froma single illuminated area DSC. In this Example, single illuminated areaSSM-DSCs with more than 2 subcells were analyzed for the first time. Theuse of multiple photoactive films under one area within the SSM-DSCframework is made possible by fine tuning the thickness of TiO₂ in eachdevice and judicious dye selection to allow for excellent lightdistribution among the films. Photovoltages (V_(oc)) ranging from1.9-4.7 V with J_(sc) from 6 to 2.4 mA/cm² and power conversionefficiency (PCE) of 7.7 to 4.1% were observed for SSM-DSCs fabricatedfrom 2-5 stacked subcells constructed with metal-free organic dyes andcobalt redox shuttles. In particular, balancing the light transmissionand absorption through TiO₂ film thickness of the individual subcellsprovided V_(oc) values per unit area as high as ˜4.7 V for a singleilluminated area device connected in series, with a PCE of ˜4.4%. Thisremarkable V_(oc) was further enhanced by series connection of twoSSM-DSCs devices irradiated side by side, which lead to an exceptionallyhigh V_(oc) of 8.5 V, J_(c) 1.9 (mA/cm²), FF of 25% and PCE of 4%.

The SSM-DSC approach permits incorporation of materials designed to usethe maximal potential energy of photons in each region of the solarspectrum. Importantly, SSM-DSCs were observed to maintain high V_(c)under low-light conditions, rendering these systems very attractive forindoor applications. Additionally, a SSM-DSC was found to have asolar-to-fuel conversion efficiency of 2% (2.7% including H₂ production)for the reduction of CO₂ to CO with IrO₂ and Au₂O₃ electrocatalysts,without an external bias. These devices have applicability in areas suchas, but not limited to, solar-to-fuel devices and portable energycharging systems from a low area solar cell.

Example 2

This Example describes the stepwise synthesis of(E)-2-cyano-3-(4-(7-(2,2′,2″,4,4″-pentakis(hexyloxy)-[1,1′:3′,1″-terphenyl]-5′-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylicacid, as illustrated in FIG. 19 .

5-bromo-2-(hexyloxy)-1,3-diiodobenzene (1)

To an oven dry, N₂ filled round bottom flask was added4-bromo-2,6-diiodophenol (5.0 g, 11.8 mmol), 1-bromohexane (778 ml, 14.1mmol), K₂CO₃ (2.4 g, 17.7 mmol) in 25 ml of DMF. The reaction mixturewas heated to 90° C. for 16 hrs. The crude product was extracted withhexanes to give a light yellow oil (5.8 g, 97% yield). ¹H NMR (500 MHz,CDCl₃) δ 7.09 (s, 2H), 3.96 (t, J=10.0 Hz, 3H), 1.94-1.91 (m, 2H),1.58-1.56 (m, 2H), 1.40-1.39 (m, 4H), 0.95 (t, J=5.0 Hz). ¹³C NMR (175MHz, CDCl₃) δ 157.7, 141.7, 1178.0, 91.3, 73.7, 31.7, 30.0, 25.6, 22.6,14.1. IR (neat, cm⁻¹) 3086.5, 3054.9, 2924.0, 2856.5, 2361.4, 2334.9,2091.2, 1994.6, 1731.5, 1542.8, 1526.4, 1460.1, 1424.6, 1372.4, 1239.4,1050.3, 985.8, 922.2, 860.0.

Synthesis of5′-bromo-2,2′,2″,4,4″-pentakis(hesxyl)-1,1′:3′,1″″-terphenyl (2)

To an oven dry, N₂ filled pressure flask was added5-bromo-2-(hexyloxy)-1,3-diiodobenzene (718 mg, 1.41 mmol), Pd(PPh₃)₄(163 mg, 0.1 mol %), (2,4-bis(hexyloxy)phenyl)boronic acid (1.0 g, 3.1mmol) in 5 ml of toluene. 1.22 ml of a 2M solution of K₂CO₃ and 3 dropsof Aliquot 336 were then added to the reaction mixture and heated to110° C. for 16 hrs. The crude product was extracted with DCM andpurified by silica gel chromatography using 10% DCM/Hex to give acolorless oil (604 mg, 53% yield). ¹H NMR (300 MHz, CDCl₃) δ 7.37 (s,2H), 7.24 (d, J=9.0 Hz, 2H), 6.54-6.49 (m, 4H), 4.00-3.93 (m, 8H), 3.18(t, J=6.0, 2H), 1.82-1.69 (m, 8H), 1.52-1.29 (m, 24H), 1.1-1.0 (m, 4H),0.96-0.88 (m, 16H), 0.78 (t, J=6.0 Hz, 3H). ¹³C NMR (175 MHz, CDCl₃) δ159.9, 157.3, 154.8, 134.1, 133.3, 132.0, 119.8, 114.6, 104.7, 100.1,72.6, 68.4, 68.1, 31.6, 31.5, 31.4, 29.8, 29.3, 29.1, 25.8, 25.8, 25.4,22.6, 22.6, 22.6, 14.1, 14.1, 14.0. IR (neat, cm⁻¹) 2924.0, 2858.2,2350.3, 2115.8, 1607.1, 1577.2, 1504.8, 1462.7, 1420.2, 1297.3, 1176.2,1113.7, 1024.3, 212.3. HRMS (ESI) m/z C₄₈H₇₃BrO₅Cs ([M+Cs]⁺) 941.3696,found 943.3630.

Synthesis of(2,2′,2″,4,4″-pentakis(hexyloxy)-[1,1′:3′,1″-terphsnyl]-5′-yl)boronicacid (3)

To an oven dry, N₂ filled round bottom flask was added 2 (200 mg, 0.12mmol) in 1 ml of anhydrous THF. The reaction mixture was cooled to −78°C. and 2.5 M n-butyl lithium (0.12 ml, 0.14 mmol) was added dropwise.The reaction was left to stir at −78° C. for 1½ hrs. Trimethyl borate(0.09 ml, 0.81 mmol) was then added dropwise to the reaction mixture andleft to cool to room temperature overnight. A 3:1 10% HCl:Et₂O was addedand the crude product was extracted with Et20 and purified by silica gelchromatography using 5% EtAc/Hex to give a light yellow oil (108 mg, 57%yield). ¹H NMR (500 MHz, CDCl₃) δ 8.03 (s, 2H), 7.26 (d, J=9 Hz, 2H),6.51-6.54 (m, 4H), 4.02 (t, J=5 Hz, 4H), 3.88 (t, J=5 Hz, 4H), 3.27 (t,J=5 Hz, 2H), 1.82-1.69 (m, 8H), 1.52-1.29 (m, 24H), 1.0-1.082 (m, 4H),0.88-0.96 (m, 16H), 0.78 (t, J=6.0, 3H). ¹³C NMR (175 MHz, CDCl₃) δ202.2, 154.7, 152.9, 133.9, 127.3, 126.8, 116.7, 99.8, 95.4, 26.9, 26.7,26.2, 25.1, 24.7, 24.6, 24.4, 21.1, 21.0, 20.8, 20.6, 17.9, 17.9, 17.7.IR (neat, cm⁻¹) 3470.2, 3377.2, 2928.1, 2861.8, 2362.1, 2160.5, 1973.2,1679.6, 1603.3, 1504.0, 1463.8, 1353.1, 1249.0, 1171.2, 1111.6, 1016.0,954.3, 12.3. IRMS (ESI) m/z C₄₈H₇₅BO₇Cs ([M+Cs]⁺) 907.4669, found907.5184.

4-(7-(2,2′,2″,4,4″-pentakis(hexyloxy)-[1,1′:3′,1″-terphenyl]-5′-yl)benzo[c][1,2,5]thiadiazol-4-yl)benzaldehyde(4)

To an oven dry, N₂ filled pressure flask was added4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)benzaldehyde (FIG. 20 ) (45 mg,0.14 mmol), 3 (109 mg, 0.15 mmol), Pd(PPh₃)₄ (16 mg, 0.1 mol %), 0.1 mlof 2 M solution of K₂CO₃ and 2 ml of (THF:toluene, 1:1). The vial wassealed and reaction mixture was stirred at 110° C. for 12 hrs. The crudeproduct was extracted with DCM and purified by silica gel chromatographyusing 20% EtAc/Hex to give a yellow oil (104 mg, 76% yield). ¹H NMR (500MHz, CDCl₃) δ 10.14 (s, 1H), 8.20 (d, J=10.0 Hz, 2H), 8.08 (d, J=10.0Hz, 2H), 7.96 (s, 2H), 7.86 (s, 2H), 7.40 (d, J=10 Hz, 2H), 6.59-6.57(m, 4H), 4.03 (t, J=10.0 Hz, 4H), 4.00 (t, J=10.0 Hz, 4H), 3.32 (t,J=5.0 Hz, 2H), 1.86-1.83 (m, 4H), 1.75-1.72 (m, 4H), 1.39-1.32 (m, 4H),1.0-1.082 (m, 4H), 0.88-0.96 (m, 16H), 0.78 (t, J=6.0, 3H). ¹³C NMR (175MHz, CDCl₃) δ 203.6.9, 191.9, 171.1, 159.7, 157.4, 154.2, 143.6, 132.4,132.3, 131.7, 130.7, 130.0, 129.8, 129.0, 127, 120.9, 104.7, 100.2,72.7, 68.5, 68.1, 34.7, 34.5, 31.7, 31.5, 29.9, 29.4, 29.2, 29.1, 25.8,25.8, 25.4, 25.3, 22.5, 21.1, 20.7. IR (neat, cm⁻¹) 2928.0, 2861.9,2361.9, 2029.3, 1940.2, 1701.7, 1636.9, 1505.8, 1461.7, 1301.3, 1178.4.HRMS (ESI) m/z C₆₁H₈₀N₂O₆S ([M+Cs]⁺) 1101.4791, found 1101.5381.

(E)-2-cyano-3-(4-(7-(2,2′,2″,4,4″-pentakis(hexyloxy)-[1,1′:3′,1″-terphenyl]-5′-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylicacid (5)

To an oven dry, N₂ filled pressure flask was added 4 (104 mg, 0.12mmol), cyanoacetic acid (270 mg, 0.32 mmol), piperidine (0.074 ml, 0.75mmol) and 2 ml of Chloroform. The vial was sealed and reaction mixturewas stirred at 90° C. for 1 hr. The crude product was extracted with DCMand purified by silica gel chromatography using DCM, 5% MeOH/DCM and100% MeOH/DCM to give a yellow oil (80 mg, 7300 yield). ¹H NMR (500 MHz,CDCl₃) δ 8.40 (s, 1H), 8.22 (ap s, 4H), 7.96 (s, 2H), 7.87 (ap s, 2H),7.39 (d, J=10.0 Hz, 2H), 6.60-6.56 (m, 4H), 4.03 (t, J=5.0 Hz, 4H), 4.00(t, J=5.0 Hz, 4H), 3.32 (t, J=5.0 Hz, 2H), 1.86-1.75 (m, 4H), 1.75-1.70(m, 4H), 1.54-1.40 (m, 4H), 1.35-1.11 (m, 28H), 0.95 (t, J=7.0 Hz 6H),0.81 (t, J=10.0, 3H), 0.76 (t, J=10.0 Hz, 6H). An absorption spectrum isillustrated in FIG. 21 .

Synthesis of Redox Shuttle:

Fe(bpy)₃ ⁺² PF₆

2,2′-bipyridine (1.85 g, 2.27 mmol) was dissolved in 15 ml of ethanoland heated to 60° C. The reaction mixture was added slowly to a stirringsolution of FeCl₂ (0.500 g, 3.95 mmol) of in 10 ml of water also at 60°C. The solution immediately turns bright red. After 1 hr. the reactionmixture was cooled to room temperature and a saturated solution of KPF₆in water was added to precipitate out the hexafluorophosphate salt. Thecomplex was collected by vacuum filtration and was bed with water andethanol to give the diamagnetic product as a bright red solid. ¹HNMR(300 MHz, Acetone-d₆) δ 8.86 (d, J=9.0 Hz, 2H), 8.29 (t, J=6.0 Hz, 2H),7.75 (d, J=6.0 Hz, 2H), 7.60 (t, J=6.0 Hz, 2H).

Fe(bpy)₃ ³⁺ PF₆

2,2′-bipyridine (1.85 g, 2.27 mmol) was dissolved in 15 ml of ethanoland heated to 60° C. The reaction mixture was added slowly to a stirringsolution of FeCl₃ (0.500 g, 3.95 mmol) of in 10 ml of water also at 60°C. The solution immediately turns brown. After 1 hr. the reactionmixture was cooled to room temperature and a saturated solution of KPF₆in water was added to precipitate out the hexafluorophosphate salt. Thecomplex was collected by vacuum filtration and washed with water andethanol to give the paramagnetic product as a light brown solid.

Example 3

This Example describes the results of Fe(bpy)₃ ^(3+/2+) concentrationstudies (3+3+5 microns TiO₂) (Table 13).

TABLE 13 Parameters for Fe(bpy) concentration studies (3 + 3 + 5microns). Cell PEDOT V_(oc) (mV) J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃ E11.18 ± 0.009 2.4 ± 0 0.64 ± 0.02 2.0 ± 0.05 2 RR-9 Fe(bpy)₃ E2 0.696 0.10.64 0.04 1 RR-9 Fe(bpy)₃ E3 1.264 2.4 0.55 1.7 1 E1 = 0.25M Fe(bpy)₃ ²⁺E2 = 0.15M Fe(bpy)₃ ²⁺ E3 = 0.35M Fe(bpy)₃ ²⁺ 0.5M Fe(bpy)₃ ³⁺ 0.05MFe(bpy)₃ ³⁺ 0.05M Fe(bpy)₃ ³⁺ (Sat. Solution) Cells were dipped inEtOH:TFH (4:1) solution for 19 hours.

Example 4

This Example describes the results of LiTFSI concentration studies(3+3+5 microns) (Table 14).

TABLE 14 Parameters for LiTFSI concentration studies (3 + 3 + 5microns). Cell PEDOT V_(oc) (mV) J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃0.05M E1 1.156 ± 0.04  2.2 ± 0.2 0.47 ± 0.07  1.2 ± 0.1  2 RR-9 Fe(bpy)₃0.5M E1 1.026 ± 0.02  1.6 ± 0.1 0.54 ± 0.007 1.0 ± 0.1  2 RR-9 Fe(bpy)₃0.1M E1  1.18 ± 0.009 2.4 ± 0   0.64 ± 0.02  2.0 ± 0.05 2 E1 = 0.25MFe(bpy)₃ ²⁺ 0.05M Fe(bpy)₃ ³⁺ Cells were dipped in 60:1 CDCA 4:1EtOH/THF solution

Example 5

This Example describes the results of solvents studies (3+3+5 microns)(Table 15).

TABLE 15 Parameters for solvents studies (3 + 3 + 5 microns). CellPEDOT/E1 V_(oc) (mV) J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃ 1.219 ± 0.042.8 ± 0.07 0.44 ± 0.03 1.6 ± 0.2 2 MeCN/ButEtOH RR-9 Fe(bpy)₃ 1.131 ±0.03 2.2 ± 0.2  0.55 ± 0.02 1.4 ± 0.2 2 EtOH/THF RR-9 Fe(bpy)₃ 1.029 ±0.03 1.7 ± 0.2  0.48 ± 0.07 0.9 ± 0.3 2 DCM/EtOH RR-9 Fe(bpy)₃ CB 1.0131.5 0.56 0.9 1 E1 = 0.25M Fe(bpy)₃ ²⁺ 0.05M Fe(bpy)₃ ³⁺

Example 6

This Example describes the results of dye concentration studies (3+3+5microns) (Table 16).

TABLE 16 Parameters for dye concentration studies (3 + 3 + 5 microns).Cell PEDOT/ # E1 V_(oc) (mV) J_(sc) FF PCE Cells RR-9 Fe(bpy)₃ 1.10161.3 0.48 0.7 1 0.1M dye RR-9 Fe(bpy)₃ 1.280 ± 0.02 2.8 ± 0   0.44 ±0.05  1.7 ± 0.2  2 0.5M dye RR-9 Fe(bpy)₃ 1.283 ± 0.03 2.8 ± 0.07 0.42 ±0.007 1.6 ± 0.01 2 1.0M dye RR-9 Fe(bpy)₃ 1.409 2.8 0.56 2.4 1 0.5M dye30 min N₂ RR-9 Fe(bpy)₃ 1.401 2.6 0.50 1.9 1 0.5M dye 30 min N₂overnight E1 = 0.25M Fe(bpy)₃ ²⁺ 0.05M Fe(bpy)₃ ³⁺ Cells were dipped inMeCN/ButOH (1:1) for 19 hours for experiment without N₂

Example 7

This Example describes the results of N₂ bubbling studies (3+3+5microns) (Table 17).

TABLE 17 Parameters for N₂ bubbling studies (3 + 3 + 5 microns). CellPEDOT/E1 V_(oc) (mV) J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃ 1.280 ± 0.022.8 ± 0   0.44 ± 0.05 1.7 ± 0.2  2 0.5M dye no N₂ 19 hour dipping RR-9Fe(bpy)₃ 1.302 2.3 0.53 1.7 1 0.5M 15 min N₂ RR-9 Fe(bpy)₃ 1.409 2.80.56 2.4 1 0.5M 30 min N₂ RR-9 Fe(bpy)₃ 1.283 ± 0.01 2.7 ± 0.2 0.51 ±0.01 1.8 ± 0.06 2 0.5M 45 min N₂ E1 = 0.25M Fe(bpy)₃ ²⁺ 0.05M Fe(bpy)₃³⁺ After bubbling N₂ all cells were let to sit in dye solution for anextra 30 minutes.

Example 8

This Example describes the results of dipping time studies (3+5 microns)(Table 18).

TABLE 18 Parameters for dipping time (3 + 5 microns) studies. CellPEDOT/ E1 V_(oc) (mV) J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃ 1.312 2.1 0.451.3 1 0.5M dye 19 hour dipping RR-9 Fe(bpy)₃ 1.224 ± 0.009 2.0 ± 0.070.55 ± 0.03 1.4 ± 0   2 0.5M 2 hours RR-9 Fe(bpy)₃ 1.401 ± 0.01  2.6 ±0.3  0.49 ± 0.02 1.8 ± 0.1  2 4 hours RR-9 Fe(bpy)₃ 1.359 ± 0.002 2.4 ±0.07 0.51 ± 0.04 1.7 ± 0.07 2 12 hours E1 = 0.25M Fe(bpy)₃ ²⁺ 0.05MFe(bpy)₃ ³⁺

Example 9

This Example describes the results of thickness studies (Table 19).

TABLE 19 Parameters for thickness studies. Cell PEDOT/E1 V_(oc) (mV)J_(sc) FF PCE # Cells RR-9 Fe(bpy)₃ 1.312 2.1 0.45 1.3 1 3 + 5 RR-9Fe(bpy)₃ 1.280 ± 0.02 2.8 ± 0   0.44 ± 0.05 1.7 ± 0.2 2 3 + 3 + 5 RR-9Fe(bpy)₃ 1.192 ± 0.07 1.4 ± 0.07 0.55 ± 0.03 1.0 ± 0.2 2 3 + 3 + 3 + 5E1 = 0.25M Fe(bpy)₃ ²⁺ 0.05M Fe(bpy)₃ ³⁺ Cells were dipped for 19 hours.Made with old dye solution.

Example 10

Introduction

Since the seminal discovered by O'Regan and Gratzel of dye sensitizedsolar cells (DSCs) in 1991, it has been a topic of extensive research.While improving the overall efficiency of the devices has been a topicof intense research throughout the years, improving the open circuitvoltage (V_(oc)) has emerged as an important topic of research. DSCs area clean energy source that with voltage improvements can be used topower a number of electrocatalysts for chemical transformations such ashydrogen production from water splitting or carbon fuels from co,reduction, and DSCs can provide a high voltage electrical source topower electronic devices. DSC devices operate through (1) the absorptionof light by the organic component (dye), (2) transfer of anexcited-state electron to the semiconductor (commonly TiO₂), (3) theelectron then travels an external circuit to a counter electrode where(4) the redox mediator collects the electron and returns it to the dyeto complete the electrical circuit. The maximum voltage provided isdefined by the energetic distance between the semiconductor conductionband and the redox shuttle electrochemical potential. A redox shuttlewith a low oxidation potential can be selected to give higher V_(oc).Tuning the redox shuttle potentials also requires novel wide band dyedesigns as the electron transfer from the redox shuttle to the dye (step4 above) to complete the circuit requires that the oxidized dyes redoxpotential be below that of the redox shuttle. Herein we report the firstFe redox mediator system to be used in a DSC cell and a novel wide bandgap dye (RR9) that together this system produces a 1.4 V cell.Furthermore, prior attempts to increase the V_(oc) required the use ofMg⁺² doped treatments to raise the CB of TiO₂, our system does notrequire any treatment on the TiO₂ CB to achieve a 1.4V cell.

Results and Discussions

FIGS. 22A-B illustrates the chemical structures of RR9 and Fe(bpy)₃^(3+/2+) redox mediator. RR9 is a simple Donor-bridge-acceptor (D-π-A)moiety dye with a donor molecule featuring 5 hydroxyl chains to preventthe non-productive recombination with the redox mediator pathway and dyeaggregation. The energy levels of RR9 were evaluated to be 1.5 V for theground state oxidation and −1.1 V for the excited-state oxidation,suitable to work with the Fe(bpy)₃ ^(3+/2+) redox shuttles that have anoxidation potential at 1.3 V (Table 20).

TABLE 20 Dye and Absorbance data^([a]) Electrochemical data Redoxmediator λ_(max) (nm) ε (M⁻¹cm⁻¹) λ_(onset) (nm)^([b]) E(_(S+/S))(V)^([c]) E(_(S+/S)) (V)^([d]) E_(g) ^(opt) (eV)^([e]) RR9 412 18480 4751.5 −1.1 Fe(bpy)₃ ^(+2/+3) 1.3

Designing dyes with energetics suitable for use at various positions iscritical in further improving the performance of SSM-DSC devices. Theseenergetic values show an encouraging initial direction where thismolecular dye design approach has been put forward. As shown in Table 21below, which compares RR9 to a benchmark dye, a record settingphotovoltage of 1.4 V was achieved through this redox shuttle-dye pair.This allows for the front cell in a DSC system to generate 1.4 V ratherthan 1.0 V in the above example and can lead to SSM-DSCs with overallperformances of >10 V for two side by side systems.

TABLE 21 Photovoltaic Parameters of DSC Devices Entry Dye (TiO₂thickness) Redox Shuttle^([a]) V_(oc) [mV] J_(sc) [mA/cm²] FF PCE [%] 1RR9 (2.7 μm) Fe 1420 2.8 0.47 1.9 2 D35 (2.7 μm) Fe 550 0.1 0.56 0.0 3RR9 (2.7 μm) Co 680 3.5 0.72 1.8 4 D35 (2.7 μm) Co 760 8.8 0.67 4.6^([a])Fe is Fe(bpy)₃ ^(3+/2+). Co is Co(bpy)₃ ^(3+/2+) unless noted. Allvalues are average of two cells.

During device optimization several parameters were evaluated, our bestresults were with MeCN/tBuOH and a minimal amount of chlorobenzene inorder to solubilize the dye in this combination of solvents (FIG. 23 ).Initially we used TiO₂ 5.3 microns thick and a 5 micron scattering layerand a 19 hrs dipping time. To further enhance the performance of thedevice nitrogen flow was applied to the system for 30 min. The flow ofN₂ causes the dye to penetrate deeper into the TiO₂ and gave an increaseof ˜100 mV on the best cell (FIG. 24 ).

A 1.4 V cell was also achieved without N₂ flowing by controlling thethickness of TiO₂. The best results were obtained with a 2.7 micronsTiO₂ film and a 5 micron scattering layer in a 0.5M solution ofMeCN/tbutOH and minimal amount of chlorobenzene with a dipping time of 4hrs (FIG. 25 ). The optimized device gave an IPCE (Incidentphoton-to-current efficiency) of ˜60% (FIGS. 26A-B).

Finally, a three subcell SSM-DSC was constructed with RR9/Fe(bpy)₃^(+3/+2) as a front cell, D35/Co(bpy-pz)₃ ^(+3/+2) as a second cell, andY123/Co(bpy-pz)₃ ^(+3/+2) as a third cell. As shown in Table 22, thisdevice achieved a voltage of 3.3 V, the highest known to date for athree tandem device system, beating the previous record of 2.8 alsogenerated by the instant inventors (FIG. 27 ). The optimization of thefront cell (RR9/Fe(bpy)₃ ^(+3/+2)) enabled an increase of 300 mA fromthe previous record.

TABLE 22 Photovoltaic Parameters of SSM-DSC Devices Entry Dye (TiO₂thickness)^([a]) Redox Shuttle^([b]) V_(oc) [mV] J_(sc) [mA/cm²] FF PCE[%] 1 RR9 (1.3 μm)/D35/Y123 Fe/Co/Co 2790 1.5 0.48 2.0 2 RR9 (1.3μm)/D35/Y123 Fe/Co*/Co* 3100 1.8 0.54 3.1 3 RR9 (1.5 μm)/D35/Y123Fe/Co*/Co* 3180 2.2 0.49 3.5 4 RR9 (1.3 μm)/Y123/Y123 Fe/Co*/Co* 33401.9 0.56 3.5 ^([a])For D35 and Y123 subcells the TiO₂ thickness is 2.2μm and 4.5 μm, respectively. ^([b])Fe is Fe(bpy)₃ ^(3+/2+). Co isCo(bpy)₃ ^(3+/2+) unless noted. Co* is Co(bpy-pz)₂ ^(3+/2+). All valuesare average of two cells.

Conclusion

A Fe(bpy)₃ ^(+3/+2) redox shuttle was successfully used as a redoxmediator in a DSC system for the first time. It's ground state oxidationpotential of 1.3 V in conjunction with a wide band gap dye RR9 (1.5 V)generated a device with a V_(oc) up to 1.42 V (FIG. 28 ). To the best ofour knowledge, this system is the highest photovoltage achieved by asingle junction DSC device without metal oxide doping. Finally, a threedevices tandem system was constructed with RR9/Fe(bpy)₃ ^(+3/+2) as afront cell, D35/Co(bpy-pz)₃ ^(+3/+2) as a second cell andY123/Co(bpy-pz)₃ ^(+3/+2) as a third cell. This tandem system achieved3.3V (FIG. 28 ), the highest voltage for a three subcell device tandemto date, beating the previous record of 2.8 V also generated by our lab.Research is under way in our lab to test this redox mediator system withnew dye designs as well as modifying the ligands on the Fe complex toincrease the V_(oc) even further. The further increase in Vo, canpotentially allow for a single junction DSC to provide enough voltage topower electrocatalytic water oxidation for the first time. Atapproximately 1.5 V water oxidation to generate hydrogen gas as a fuelbecomes possible for the first time. This strategy will certainly leadto materials with these capabilities.

Example 11

This Example is directed to development of novel dyes designed tomaximize the voltages of specific subcells in SSM-DSC devices. A rangeof dyes with excited-states near −0.7 V and a variable array ofground-state potentials (0.3-2.4 V vs NHE) are highly desirable.Accordingly, several dyes with energetics nearing these values have beensynthesized and tested in DSC devices. Through optimizations and the useof a novel dye structure (FIGS. 29-31 ), voltages exceeding 10V arepossible from a single illuminated area SSM-DSC device.

Dyes AP11, AP14, AP16 and AP17 (FIG. 29 ) show excellent energy levelcontrol with excited-states near ideal. Additionally, the ground-statepotentials of these dyes were modulated, as seen in FIG. 32A and cyclicvoltammetry studies. The oxidations potentials of these dyes are shownin FIG. 32B. Furthermore, these dyes exhibited excellent performance inDSC devices with regard to high photovoltages nearing the record setting1.4V reported herein. FIGS. 33A-B and Table 23 detail the performancesof these devices.

TABLE 23 Cell V_(oc) (V) J_(sc) (mA/cm²) FF (%) PCE (%) AP11 1.20 3.00.48 1.7 AP14 1.24 2.8 0.49 1.9 AP16 1.33 3.3 0.45 2.0 AP17 TBA TBA TBATBA

A schematic of a method for synthesizing various donor groups, includingthose of dyes AP11, AP14, AP16, and AP17, is shown in FIG. 34 . FIG. 35shows a schematic of a method for synthesizing dyes including the donorgroups of FIG. 34 .

As illustrated in Examples 10-11, the novel dye design and low energyredox shuttle pairing can dramatically improve the record high voltagesobserved in Examples 1-9. Examples 10 and 11 offer a novel path forwardin potentially powering higher voltage systems with low surface areassuch as laptops and transport vehicle electronics.

Example 12

This Example is directed to development of novel devices incorporatingantireflective coatings and/or immersion oils. More specifically, asillustrated in FIG. 36 , the SSM-DSC systems discussed hereinabove weremodified to further include antireflective coatings and/or immersionoils. Referring to FIG. 37 , one such antireflective coating includesCYTOP. The dyes used in this Example included D35 and Y123, both ofwhich are discussed above, as well as B11, which is shown in FIG. 38 .

The use of an antireflective coating and/or an immersion oil changed theinterfacial refractive index to minimize light losses. This use ofantireflective coating and/or immersion oil provided an overall powerconversion efficiency (PCE) of greater than 10% with output of greaterthan 2 V (Table 24). The PCE of more than 10% represents a substantialincrease as compared of the 3 SSM-DSC systems not includingantireflective coatings and/or immersion oils.

TABLE 24 Performance of Devices Including Antireflective Coatings andMatched Refractive Index Oils. Thickness V_(oc) J_(sc) Device (μm) (mV)(mA/cm²) FF % PCE % 2 Subcell SSM-DSC Devices Y123/B11 2.2/10 1560 7.90.64  7.9 Y123/B11 CYTOP 2.2/10 1550 9.3 0.60  8.7 3 Subcell SSM-DSCDevices D35/Y123/B11 1.5/2.2/10 2500 4.8 0.54  6.5 D35/Y123/B11 CYTOP1.5/2.2/10 2500 5.4 0.66  8.6 D35/B11/B11 1.5/2.2/10 2320 4.9 0.75  8.6D35/B11/B11 CYTOP 1.5/2.2/10 2280 5.5 0.71  9.3 D35/B11/B11 1.5/2.2/102290 5.7 0.76 10.1 CYTOP/OIL

FIG. 39 illustrates the IV potential of the D35/B11/B11, D35/B11/B11CYTOP, and D35/B11/B11 CYTOP/OIL 3 subcell SSM-DSC devices in Table 24.As shown therein, each of the devices has a similar potential (V_(oc)).However, the CYTOP and CYTOP/OIL devices have a higher initial currentdensity (J_(sc)) than the device without CYTOP or oil, while the devicewith just CYTOP has the quickest initial decline in current density.

Example 13

Mechanically stacked single illuminated area sequential seriesmultijuntion dye sensitized solar cells (SSM-DSCs) are compared with thereplacement of bottom DSC with silicon (Si) and GaAs solar cells. ThisExample illustrates the practical advantage of using highly efficientnear-infrared (>750 nm) photon-to-electron conversion efficiency of Siand GaAs cells compared to DSCs. For a three subcell SSM-DSC, top andmiddle devices were DSCs with D35 and B11 benchmark sensitizers. Acomparison of three near-infrared active DSCs, Si and GaAs as a bottomdevice revealed that D35/B11/GaAs outperforms other combinations. Powerconversion efficiency (PCE) order was D35/B11/GaAs>D35/B11/BlackDye>D35/MK2/AP25+D35>D35/B11/B11>D35/B11/Si. It was found that as abottom device under filtered or reduced illumination GaAs and DSCsresponse was superior than Si.

Solar cells are one of the major contenders for reliable and wide spreadapplication of sustainable energy for future needs. Solar cellsapplication for electricity generation is an attractive and sustainableoption due to its renewable nature, mobile accessibility, abundance, andcost advantages. To harvest and utilize the energy from sun, variousstrategies based on photovoltaic (PV), photoelectric, photocatalytic andphotoelectrochemical effect has been explored. A PV system or solarcells mainly demonstrates the following three attributes 1) lightabsorption and charge generation 2) separation of opposite charges 3)separate extraction of charges. Among the most studied PV materials forsolar cells are semiconducting compounds from the group three and five(III-V) such as silicon, germanium, and gallium arsenide. In thisregard, the development of solar cells can be divided into threegenerations envisioned and first reported by 6% crystalline siliconsolar cell (Si) by Bell labs in 1954. Si have reached certifiedefficiencies of 20% and 26.6% for single crystal solar cells and occupymore than 90% of market share today. Si solar cells are unique inefficiently harvesting low energy photons up to 1200 nm. The secondgeneration thin-film solar cells consist of thin film semiconductingmaterials and have certified PCEs of 28.8%, 22.1%, and 22.6% for GaAs,CdTe, and CIGS, respectively. Third generation solar cells with thepotential for lower processing costs are perovskite solar cells (PSCs,25.2%) and dye-sensitized solar cells (DSCs, 13%). Among the thirdgeneration PVs, dye-sensitized solar cells (DSCs) have emerged as acommercially viable mass producing technology owing to solution processability, roll to roll production, variety of color (blue to red), lowlight performance and precious metal free materials applications.

For DSCs, to increase PCE one possible strategy is to decrease voltageloss in the short wavelength range and optical loss in the longwavelength range. Further, application of tandem (for two devices) andmultijuntion (>2 devices) configuration employing solar cells withdifferent wavelength responses can result in higher performance.ENEEF20_ENREF_17A recent and unique development of series connected DSCsis SSM-DSCs (sequential series multijunction-DSCs). SSM-DSCs aremechanically stacked single illuminated area series connected DSCs withmore than two devices in the stack. With SSM-DSCs, overall higher PCEthan any of the individual devices (>10%) in the stack and highphotovoltage (>2.0 V) can be achieved. SSM-DSCs with >2.0 V photovoltageoutput can power H₂O splitting and CO₂ reduction, thus potentiallyaddressing the intermittent nature of solar resource by storing theenergy in the chemical bonds.

Owing to highly modular configuration of SSM-DSCs (each device in thestack can be individually optimized), NIR active PV systems such assilicon and GaAs (FIG. 40 ) offer a potentially attractive alternativeto DSCs as a bottom device (>3 devices in the stack). Previously,introduction of Si, GaAs and CIGS solar cells as a bottom device hasbeen explored by Gratzel et al and others in tandem with DSCs (top andbottom devices only). Since SSM-DSCs can employ more than two devices,so offering the opportunity to further enhance the overall PCE. Thus,the basis of this study (FIG. 40 ) where the third (bottom) cell wasreplaced with either Si, GaAs or DSC devices to study the overall effecton PCE and other photovoltaic characteristics.

FIG. 40 (left) presents the concept of SSM-DSCs in regard to this study.In three subcell SSM-DSCs, photocurrent response in the stacked devicesis controlled by the choice of the sensitizer, thickness of TiO₂, redoxshuttle and reflection loss minimization. Such as an orange-yellow dyefor the front device (550-600 nm absorption onset), red dye for themiddle (650-750 nm absorption onset) and an NIR active sensitizer forthe bottom device (FIG. 40 , right). In the realm of DSCs, sensitizerswhich absorb and efficiently convert photons-to-electrons in the rangeof 400-750 nm are extensively studied and reported. However, sensitizerswith efficient photo-to-electron conversion of low energy photons (>750nm) have limited examples in the literature, owing to highly tunedrequirements for the sensitizers design. Contrary to this, silicon hasnearly ideal band gap (Eg=1.12 eV) to harvest photons far into NIRspectrum (FIG. 40 , right). However, silicon solar cells offer a lowervoltage 0.5-0.6 V, due to narrow bandgap and higher recombination lossesand require energy intensive manufacturing. Due to efficient and broadphotocurrent response introducing silicon in place of DSC as a bottomdevice thus offers an attractive option. Additionally, GaAs solar cellsoffer (1.42 eV) close to optimum bandgap and higher optical absorptioncoefficients thus resulting in higher efficiency thin film devices (FIG.40 , right). ENREF_44 To realize the practical advantage in threesubcell multijuntion configuration DSC, GaAs and Silicon solar cellshave been tested and compared as a bottom device.

Before analysing SSM-DSCs, single device power conversion efficiency(PCE) and IPCE was studied (FIG. 41 , Table 25). PCE of a single deviceand SSM-DSCs was calculated according to the equationPCE=(J_(sc)×V_(oc)×FF)/I₀ where, J_(sc) is the short-circuit currentdensity, V_(oc) is the open-circuit voltage, FF is the fill factor andI₀ is the intensity of the incident light (1 sun, air mass 1.5 G). Forthe final DSC two metal complex based B11 and Black Dye and organic dyebased co-sensitized AP25/D35 sensitizers were compared (FIG. 41 , Table25). For DSCs, all the dyes show broader light harvesting ability withBlack Dye and AP25/D35 up to 100 nm red shifted in the IPCE response(900 nm onset vs. 800 nm). IPCE max of 82% at 650 nm for Black Dye and550 nm for B11 was achieved (FIG. 42 ), whereas AP25/D35 achieved >80%IPCE response from 400-750 nm range. ENREF_45 Owing to broader currentresponse, AP25/D35 translated into impressive J_(sc) of 23.7 mA/cm²compared to 20.8 mA/cm² for Black Dye and 17.5 mA/cm² for B11. However,due to lower V_(oc) of AP25/D35 overall PCE (8.3%) was slightly lowerthan Black Dye and B11 (9.5% and 8.5%, respectively).

TABLE 25 Comparison of device performances given in FIG. 42. Devices*J_(sc) (mA/cm²) V_(oc) (mV) FF (%) PCE (%) B11 17.5 ± 1.0 686 ± 10 70 ±3  8.5 ± 0.2 Black Dye 20.8 ± 0.4 644 ± 7  70 ± 1  9.5 ± 0.1 AP25/D3523.7 ± 0.7 570 ± 11 63 ± 1  8.3 ± 0.2 GaAs 27.0 ± 0.6 824 ± 12 70 ± 315.9 ± 0.2 Si   41 ± 0.1 415 ± 1  47 ± 2  7.9 ± 0.2 *Details of TiO₂film thickness, dipping solvent, electrolyte concentrations, CYTOP anddevice assembly are given in the SI.

GaAs's overall efficient response with IPCE of >90% at 700 nm, (J_(sc)of 27 mA/cm², V_(oc) of 824 mV and ff of 70%) resulted in PCE of 15.9%,which was higher than any single device tested for this study. Comparedto photocurrent of other devices (DSCs and GaAs), silicon solar cellsJ_(c) of 41.2 mA/cm² was higher than any other device due to its broadIPCE response (FIG. 41 , bottom), whereas overall PCE was lower at 7.9%due to low V_(oc) and fill factor (Table 25).

For SSM-DSCs, front and middle device employed D35, B11 and MK2benchmark sensitizers with matching current in the stack as reported inprevious studies. In SSM-DSCs sensitizers for individual devices areselected based on their complementary absorption profile such as D35 asa wideband gap orange dye (absorption onset (λ_(onset)), 570 nm,λ_(max)=500 nm), and the 2^(nd) device B11 (λ_(onset)=680 nm,λ_(max)=554 nm) and MK2 (λ_(onset)=560 nm, λ_(max)=480 nm) as a red dyefor a fully organic SSM-DSC configuration. Owning to wideband gap of thefront device (D35), it can be paired with a low lying redox shuttle suchas Co(bpy-pz)₂ ^(3+/2+) (bpy-pz=6-(1H-pyrazol-1-yl)-2,2′-bipyridine,redox potential=0.86 V vs. NHE). This strategic coupling of an orangedye and redox shuttle can lead to theoretical maximum photovoltage of1.36 V with TiO₂ conduction band (CB) value at −0.5 V vs. NHE (assumingno shift in the conduction band of TiO₂). TiO₂ thickness of 1.8 μm, 5.0μm, and 3.5 μm was optimized for D35, B11 and MK2 based DSCsrespectively, with optimum matching of current for the front two devicesin a three cell SSM-DSC. Thus, the choice of 2^(nd) subcell dye andthickness depends on the thickness of the front cell TiO₂ and the dye,and breadth and onset of IPCE response of the third cell (for threesubcell SSM-DSCs). However, the final (3^(rd) cell) dyes tested inSSM-DSCs so far were limited mainly in current generation to 900 nm.

FIG. 42 and Table 26, summarizes the results for multijuntion devices(SSM-DSCs configuration) with three subcells stacked on top of eachother. First comparing all the DSC based SSM-DSCs, D35/B11/B11 (in theorder from top to bottom), D35/B11/Black Dye and D35/MK2/AP25+D35.Impressively, all the DSC based SSM-DSCs resulted in PCE higher than anysingle device in the stack (Table 25 and 27). D35/B11/Black Dye SSM-DSCsoverall performance was superior to other two DSC only SSM-DSCs. ForD35/B11/B11, the thickness of 1^(st) and 2^(nd) device (1.5 μm and 2.2μm) was lesser than the 1^(st) and 2^(nd) device for D35/B11/Black Dyeand D35/MK2/AP25+D35 (Table 26) to achieve current matching. Contrary tothis, thicker 1^(st) and 2^(nd) devices were employed for D35/B11/BlackDye and D35/MK2/AP25+D35 because the bottom device can now harvest NIRphotons (900 nm IPCE onset instead of 800 nm of B11). Overall higher PCEof D35/B11/Black Dye (10.5%) compared to D35/MK2/AP25+D35 (9.2%) was dueto higher photovoltage and fill factor of farmer (Table 25, FIG. 42 ).AP25+D35 itself employ higher Li⁺ (loading), which is known to lowerV_(oc) due to positive shift in TiO₂ conduction band.

TABLE 26 Summary of DSCs device data for different SSM-DSCs.* J_(sc)Devices Thickness (μm) V_(oc) (mV) (mA/cm²) FF (%) PCE (%) D35/B11/B111.5 μm/2.2 μm/ 2281 ± 20 5.5 ± 0.1 71 ± 2  9.3 ± 0.1 full D35/B11/ 1.8μm/5.0 μm/ 2323 ± 54 6.5 ± 0.4 66 ± 3 10.5 ± 0.1 Black Dye full D35/MK2/1.8 μm/5.0 μm/ 2132 ± 21 6.4 ± 0.4 64 ± 2  9.2 ± 0.3 AP25 + D35 fullD35/B11/ 1.8 μm/5.0 μm/ 2472 ± 27 6.9 ± 0.3 68 ± 3 11.2 ± 0.1 GaAs fullD35/B11/Si 1.8 μm/5.0 μm/ 2064 ± 5  6.7 ± 0.2 67 ± 2  9.1 ± 0.1 full*Details of dipping solvent, electrolyte concentrations, CYTOP anddevice assembly are given in the SI. “full” refers to the bottom cell.

With the highest performing GaAs device as the bottom device, theoverall PCE (11.2%, Table 26) for the stacked series connected deviceswas higher than any other combinations tested for SSM-DSCs in thisstudy. However, the D35/B11/GaAs (11.2%) SSM-DSC device performance wasnot higher than the single GaAs (15.9%) and other device (DSCs) in thestack.

Previous highest PCE with the mechanical stacking of a DSC and GaAs is7.63% and 6.87 in monolithic configuration. In that regard, DSC/DSC/GaAsconfiguration was found with impressively high V_(oc) of 2.5 V and PCEof 11.2%. With D35/B11/Si mechanical stacking configuration, wellmatched current was achieved however overall PCE (9.1%, Table 25, FIG.41 ) was lowest compared to other combinations studied. This is mainlycaused by the lower V_(oc) and fill factor of individual Si solar cell(Table 25) which also led to SSM-DSC (D35/B11/Si) with lower V_(oc) andfill factor (Table 26).

To further elaborate the results for SSM-DSCs in Table 26, performanceof individual bottom devices (final 3^(rd)) under filtered light (i.e.,in the stack) was compared (FIG. 43 , Table 27) in terms of IPCEresponse.

TABLE 27 Summary of 3^(rd) device performance when in the stack ofdevices for SSM-DSCs (under filtered light).* J_(sc) Device V_(oc) (mV)(mA/cm²) FF (%) PCE (%) B11 658 ± 7   5.4 ± 0.1 77 ± 1 2.8 ± 0.1 BlackDye 612 ± 3   6.7 ± 0.2 78 ± 1 3.4 ± 0.1 AP25/D35 567 ± 11  6.2 ± 0.1 77± 1 2.7 ± 0.1 GaAs 732 ± 4   7.6 ± 0.2 71 ± 1 3.9 ± 0.2 Si 310 ± 2  13.7± 0.4 42 ± 1 1.7 ± 0.1

For all DSC based SSM-DSCs, performance of Black Dye as bottom devicewas superior to B11 and AP25/D35. J_(c) of B11 as 3^(rd) device wasmainly limited due to lack of light absorption breadth and blue shiftedIPCE onset (FIG. 43 ). Lowered PCE of AP25/D35 (2.7%) compared to BlackDye (3.45) was due to lower V_(oc) mainly as discussed already. GaAs'sperformance as the 3^(rd) device was superior than any other device inthe solar cells studied here. This can be assigned to its highlyefficient red photon harvesting ability (FIGS. 41 and 43 ) and higherV_(oc). Si solar cell as the bottom device maintained the high enoughJ_(c) (13.7 mA/cm², Table 27) due to broad IPCE response (FIG. 43 ,Table 27), however, inherently lower V_(oc) and fill factor resulted inlowest performance 3^(rd) device. Interestingly, the order of individualdevice PCE performance (Table 25, GaAs>Black Dye>B11>AP25/D35>Si) wassame as when the devices were studied under filtered light in the stack(Table 27). Since Si is observed to maintains high enough photocurrentunder filtered light for three cell SSM-DSCs, it was further studied asbottom device in four (D35/Y123/Black Dye/Si) and two devices (B11/Si)configuration. Thicker B11 (5.5 μm) as top and GaAs as bottomoutperformed B11/Si (11.5% vs. 10.3), respectively. With four cellSSM-DSCs (D35/Y123/Black Dye/Si), bottom Si cell showed well matchingcurrent with other devices in the stack, however, V_(oc) was low as 2.7V and PCE of 8.5%.

FIG. 44 further illustrates the reason for lower performance when Sisolar cell was employed as the bottom device. Inherently, DSCs showedenhanced performance under reduced light due to diminished aggregationrelated losses and efficient handling of absorbed photons, thus makingDSCs an ideal choice for indoor solar cells and bottom devices inSSM-DSCs. ENREF_46 GaAs too maintains its performance under loweredillumination with minimal loss in overall PCE (FIG. 44 ). It isnoticeable (FIG. 44 ), that the solar cell performance difference becomemore evident after 40% sun value, under which condition organic dyebased DSC (AP25/D35) outperform Black Dye, GaAs and Si cells.

Conclusions

In summary, SSM-DSCs were successfully engineered with a comparison ofDSC, Si and GaAs bottom cells. Impressively, SSM-DSCs performance washigher than any single device in the stack only in the case of all DSCs(highest PCE of 10.5% from D35/B11/Black Dye). By replacing Black DyeDSC with a highly efficient GaAs solar cell in a three cell SSM-DSC,highest PCE was achieved in the series (11.2% vs. 10.5%), respectively.Combing the inherently high photovoltage from DSCs and GaAs in a seriesmultijuntion device led to impressively high V_(oc) of 2.5 V. Si solarcells were found to be poor in performance compared to DSCs and GaAs asbottom device due to lower photovoltage and ff. As a 3^(rd) device,Black Dye DSCs (PCE 3.4%) was slightly lower in performance compared toGaAs (3.9%).

Example 14

In initial investigations into mechanically stacked DSC devices, thelimit was 2 subcell (termed ‘tandem’) due to the prevailing theory inthe field that photon losses were a result of scattering of light,reflectance of light, and poor transmission which would be too great fora third subcell to provide significant device performance increases.However, this circumvents a substantial advantage of the mechanicallystacked approach relative to other options such as dichroic mirrors forbeam splitting or monolithic approaches which are traditionally limitedto 2 subcell tandem designs due to fabrication technique complexity orexperimental setup complexity. By stacking dye-sensitized subcells inseries to create an illuminated area where each subcell filters photonsfor subsequent active layers, the present inventors discovered that theywere able to form a three subcell arrangement for a multijunction solarcell. This mechanically stacked approach is uniquely suited to allow formultijunction systems at 3 or greater subcells with a simple fabricatedesign.

This discovery shows a surprising performance enhancement by utilizing athird subcell. It is observed that a 3 subcell D35/Y123/HD-2-mono (I)device mechanically stacked DSC significantly increases in performance(7.7% PCE in the first example reported) when properly configuredrelative to 2 subcell D35/Y123 devices (7.1% PCE in the first examplereported) and >3 subcell D35/D35/Y123/Y123 or D35/D35/Y123/Y123/Y123devices (5.8% PCE or less). The design of the three subcell deviceallows for better division of photons between active layers, which canlimit thermal losses by efficiently utilizing the potential energy ofphotons in a specific region. This is achieved by specifically tailoringdyes to absorb distinct regions of photons desired for these threesubcell multijunction SSM-DSC devices. When RR9 is used as the frontsubcell, the V_(oc) per subcell rises to a record high 1.12 V persubcell with RR9 (Fe)/Y123/Y123 over the 0.96 V per subcell reported forthe 2 subcell D35/Y123 device and 0.88 V per subcell for the threesubcell D35/Y123/HD-2-mono (I) device. The voltage per subcell is anindicator of the amount of thermal energy waste in the system.Theoretically, in the absence of photon loss pathways due to incompletetransmission, infinite subcells could give a thermal loss-free system.However, practically this is not achievable due to photon loss pathways.Both photon division between subcells and photon loss pathways must bebalanced for the highest functional practical system. This balance,prior to this technology, was believed to be at 2 subcells in thedye-sensitized solar cell field. However, though careful control of TiO2layer thickness, dye absorption breadth, use of anti-reflectivecoatings, and use of immersion oil between subcells, optical losses canbe minimized revealing that a 3 subcell device can give superiorperformance to a 2 subcell device for the first time (Table 28).

J_(SC) PCE Device Subcells V_(OC) (V) (mA/cm²) FF (%) D35/Y123 2 1.925.9 0.62 7.1 D35/Y123/HD-2-mono (I) 3 2.63 3.9 0.74 7.7D35/D35/Y123/Y123 4 3.72 2.7 0.58 5.8 D35/D35/Y123/Y123/Y123 5 4.67 2.40.38 4.3 RR9 (Fe)/Y123/Y123 3 3.34 1.9 0.56 3.5 D35/B11(I)/B11(I) 3 2.285.5 0.71 9.3 D35/B11(I)/B11(I) 3 2.29 5.7 0.76 10.1 CYTOP/OilD35/B11(I)/Black Dye (I) 3 2.27 7.1 0.72 11.1 CYTOP/Oil B11 (I) 1 0.6917.5 0.70 8.5 Black Dye (I) 1 0.64 20.8 0.70 9.5 Silicon 1 0.42 41.00.47 7.9 D35/B11 (I)/Si 3 2.06 6.7 0.67 9.1The redox shuttle is noted in parenthesis with (Fe) being an iron redoxshuttle, (I) being an iodide redox shuttle, and a cobalt based redoxshuttle is used if not noted.

While solar cells traditionally use the thickest active layer needed tocatch all light, the present inventors have surprisingly found thatthinner active layers, such as transparent or partially transparentfilms, provide improved results. In other words, the present inventorshave found that controlling the TiO₂ active layer thickness throughpaste formulations and screen mesh selection unexpectedly providesimproved performance.

SSM-DSC device performance can further be enhanced by a more carefulbalancing of photon flux through dye selection with D35/B11(I)/B11(I)leading to a 9.3% PCE. This performance can be enhanced significantly to10.1% through the use of an antireflective coating (CYTOP) and anemersion oil designed to limit photon reflection or diffraction lossesbetween subcells. The highest tested performance to date is observedwith a D35/B11(I)/Black Dye (I) CYTOP/Oil SSM-DSC device at 11.1% PCE,which is made possible by controlling TiO2 thicknesses of each layer toevenly divide photons through three complementary chromophores thatabsorb high (D35), medium (B11), and lower energy photon (Black Dye). Ifthe TiO2 thickness is not carefully controlled, the subcell photon fluxis unbalanced leading to lower device performance typically on the orderof <9% PCE. The high performing D35/B11(I)/Black Dye (I) SSM-DSC deviceis significantly greater performing than the individual dyes at 6.6%,8.5%, and 9.5%, respectively. This combined overall better performanceis not expected to occur without exceptional photon flux balance madepossible by dye selection, TiO2 thickness control, anti-reflectivecoatings, and immersion oils-all of which are necessary to reach a peakperformance. As a demonstration of the complementary nature of thisdesign approach with that of existing solar cell technologies, a 7.9%PCE silicon solar cell was used as the third subcell in a hybridSSM-DSC/inorganic solar cell device to give 9.1% PCE. This is an initialexample of how this technology can be combined with solar cells alreadyunder mass production to boost efficiencies with existing solar cells.

This current solar cell technology can most directly be improved at thefront/first subcell. Without wishing to be bound by theory, it isbelieved that the front subcell is practically capable of >2.0 V outputwith a 500 nm absorption onset dye, with the second subcell capable of˜1.3 V output with a 650 nm onset, and an output of 0.7 V for the thirdsubcell. In the D35/B11(I)/Black Dye (I) device the subcells give 1.1 V,0.7 V, and 0.6 V, respectively. However, by replacing the first subcellwith a wide band-gap dye to give 2.0 V, the second subcell can bereplaced with D35 or Y123 which already give up to 1.2 V. This wouldgive a wide band-gap dye/D35 or Y123/Black Dye device with only 0.2 Vloss versus the 1.4 V loss of a D32/B11/Black Dye device. Therefore, theuse of a wide band-gap dye would lead to lower photon losses with thePCE linearly scaling to 19.4% PCE which is exceptionally high for asimple to mass produce technology. By comparison, commercially massproduced silicon devices are often on the order of 10-15% depending onthe silicon grade used.

With this in mind, a suitable model chromophore series has beenidentified for the front subcell dyes. The suitability of thesechromophores as front subcell systems is evidenced in their reportedground state and excited state potentials. The current record holdingdye RR9 for a front subcell device with minimal loss has ground andexcited state values of 1.56 V and −1.08 V respectively versus normalhydrogen electrode (NHE). To function efficiently in an SSM-DSC device,the ground state should be >2.0 V and the excited state should be at2.2V. CC1-CC3 all show >2.0V at up to 2.2 V. These dyes are nearlyideally position energetically to be front subcells in DSC devices.Additionally, the dyes show below incorporate long alkyl chains to limitinternal charge losses due to redox shuttle-TiO2 electron recombinationlosses.

This approach provides the desired dye for a 3 subcell (or 2 subcell orDSC-inorganic hybrid) high performing system. This design also enablesthe use of alternate positively charged redox shuttles to Fe which mustbe replaced with the RR9 devices due to strong light absorption and longterm stability concerns.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described below in detail. Itshould be understood, however, that the description of specificembodiments is not intended to limit the disclosure to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

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What is claimed is:
 1. A dye, comprising: an electron deficient acceptormoiety; a medium electron density π-bridge moiety; and an electron richdonor moiety comprising a biaryl, a substituted biaryl, or a moietyhaving the following structure:

wherein each of R¹, R², and R³ independently comprises H, alkyl, aryl,multiaryl, fused aryl, fused multiaryl, alkyl substituted aryl, alkoxysubstituted aryl, alkyl substituted multiaryl, alkoxy substitutedmultiaryl, OR⁴, or a combination thereof; and wherein each R⁴independently comprises H, alkyl, aryl, alkyl substituted aryl, alkoxysubstituted aryl, or a combination thereof.
 2. The dye of claim 1,wherein the donor moiety comprises:

and wherein n is between 1 and
 30. 3. The dye of claim 1, wherein thedye comprises:


4. The dye of claim 1, wherein the substituted biaryl comprises an alkylor alkoxy substituted biaryl.
 5. The dye of claim 4, wherein the alkylsubstituted biaryl comprises the structure:

and wherein n is between 1 and
 30. 6. The dye of claim 4, wherein thealkoxy substituted biaryl comprises the structure:

and wherein n is between 1 and
 30. 7. The dye of claim 1, wherein theπ-bridge moiety comprises:

or derivatives thereof; wherein each X independently comprises H, F, CN,or CO₂R; and wherein each R independently comprises H, alkyl, aryl,alkyl substituted aryl, alkoxy substituted aryl, or a combinationthereof.
 8. The dye of claim 1, wherein the acceptor moiety comprises:

or derivatives thereof; and wherein each R independently comprises H,alkyl, aryl, alkyl substituted aryl, alkoxy substituted aryl, or acombination thereof.
 9. A sequential series multijunction dye-sensitizedsolar cell comprising: three dye-sensitized subcells mechanicallystacked and connected in series, each of the subcells comprising; aglass substrate coated with a transparent conductive film; adye-sensitized active layer; and a redox shuttle; wherein thedye-sensitized active layer of at least the first subcell is sensitizedwith a dye comprising the dye of claim 1, D35, B11, Y123, or Black Dye.10. The dye-sensitized solar cell of claim 9, further comprising a UVcutoff filter.
 11. The dye-sensitized solar cell of claim 9, wherein thedye-sensitized active layer of the first subcell has a thickness ofbetween 0.8 and 3.0 m.
 12. The dye-sensitized solar cell of claim 11,wherein the dye-sensitized active layer of the second subcell has athickness of between 1.5 and 5.0 m.
 13. The dye-sensitized solar cell ofclaim 9, further comprising an antireflective coating applied over anon-conductive side of at least one of a photoanode and a cathode of oneor more of the dye-sensitized solar cells.
 14. The dye-sensitized solarcell of claim 9, further comprising an immersion oil positioned betweenat least two of the subcells.
 15. The dye-sensitized solar cell of claim9, wherein the dye-sensitized active layer of the first subcell issensitized with a dye comprising:


16. The dye-sensitized solar cell of claim 9, wherein the dye-sensitizedactive layer of at least the first and second subcells is sensitizedwith a dye comprising the dye of claim 1, D35, B11, Y123, or Black Dye.17. The dye-sensitized solar cell of claim 9, wherein the dye-sensitizedactive layer of each of the subcells is sensitized with a dye comprisingthe dye of claim 1, D35, B11, Y123, or Black Dye.
 18. A sequentialseries multijunction dye-sensitized solar cell comprising: at least twodye-sensitized subcells mechanically stacked and connected in series,each of the subcells comprising; a glass substrate coated with atransparent conductive; a dye-sensitized active layer; and a redoxshuttle; and at least one additional subcell mechanically stacked andconnected in series; wherein the at least one additional subcell is aninorganic solar cell.
 19. The dye-sensitized solar cell of claim 18,wherein the dye-sensitized active layer of the first subcell has athickness of between 0.8 and 3.0 μm.
 20. The dye-sensitized solar cellof claim 19, wherein the dye-sensitized active layer of the secondsubcell has a thickness of between 1.5 and 5.0 μm.